Being Well-Born: An Introduction to Eugenics
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Title: Being Well-Born: An Introduction to Eugenics
Author: Michael F. Guyer
Editor: M. V. O'Shea
Release date: May 21, 2012 [eBook #39751]
Language: English
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BEING WELL-BORN
BEING WELL-BORN
AN INTRODUCTION TO EUGENICS
_By_
MICHAEL F. GUYER, PH. D.
Professor of Zoology, The University of Wisconsin
Childhood and Youth Series
_Edited by_ M. V. O'SHEA
Professor of Education, The University of Wisconsin
INDIANAPOLIS
THE BOBBS-MERRILL COMPANY
PUBLISHERS
COPYRIGHT 1916
THE BOBBS-MERRILL COMPANY
PRESS OF
BRAUNWORTH & CO.
BOOKBINDERS AND PRINTERS
BROOKLYN, N. Y.
TO MY WIFE
HELEN M. GUYER
EDITOR'S INTRODUCTION
The writer recalls that when he was a young boy, he heard the grown-up
people in the community earnestly and incessantly debating the question:
Does heredity play a greater part in shaping one's mind and body than does
his environment? From that day to this he has listened to men and women in
every walk of life discussing the relation of heredity to environment in
determining human traits. Teachers and parents are constantly asking: "Are
such and such characteristics in my children due to their inheritance or
to the way they have been trained?" Students of juvenile delinquency and
of mental defect and deficiency are searching everywhere for light on this
matter. It is not to be wondered at that practically all people are
peculiarly interested in this problem, since it concerns intimately one's
personal traits, and it constantly confronts any one who is responsible
for the care and culture of the young.
It is suggestive to note how people differ in their views regarding the
extent to which a child's physical and mental qualities and capacities are
fixed definitely by his inheritance. The writer has often heard students
in university classes discuss the subject; and their handling of the
problem has shown how superficially and even superstitiously most persons
regard the mechanism and functions of heredity. It is significant also to
observe what extreme views many people hold regarding the possibility of
affecting a child's traits and abilities by subjecting him to specific
influences during his prenatal life. In any group of one hundred persons
chosen at random, probably seventy-five will believe in specific prenatal
influence. Many of them will believe in birthmarks due to peculiar
experiences of the mother. A popular book recently published asserts among
other things that if a mother will look upon beautiful pictures and listen
to good music during the prenatal period of her child, the latter will
possess esthetic traits and interests in high degree. On the other hand,
people generally do not seem to think that degenerate parents beget only
degenerate children. Alcoholics, feeble-minded persons and the like are
permitted to bring children into the world.
Very few people have any precise knowledge of the mechanism of heredity.
The whole thing is inscrutable to them, and is shrouded in mystery.
Superstition flourishes among even intelligent persons in respect to
heredity, and errors due to education, and tragedies resulting from
vicious social organization are all alike ascribed to its uncontrollable
forces. Most people are none the wiser because they do not know to what
extent the physical and mental defects and deviations of individuals are
due to inheritance or to the malign influences of the individual's
environment and training.
Professor Guyer, who has studied the whole problem in a thoroughgoing,
scientific way, has prepared this book with a view to illuminating some of
the mysteries that surround the subject of heredity, and to dispelling
the illusions that persist regarding it. He shows the method which nature
follows in the development of the individual. He presents the laws which
have become established respecting the extent to which and the manner in
which immediate and remote ancestors contribute to the child's physical
and mental organism. He answers many questions which those who are engaged
in social work or in education in the home or the school are asking
to-day. He discusses subjects upon which every serious-minded person
wishes to be informed. He has thus made a book which is both of
theoretical and of practical interest.
He has written in a style which should make his book attractive to the
parent and the teacher as well as to the student of the complicated
mechanism of inheritance. Only a few special terms are used, and these
should not give any reader trouble, because the treatment throughout is so
concrete that the meaning of the terms will be easily grasped. Further,
the book is illustrated, with many attractive and instructive
illustrations which will show at a glance the working of the principles of
inheritance which are developed in the text.
This book may be heartily commended to all who are interested in questions
of human nature, education and social reform. It should enable the parent,
the teacher and the legislator to understand more clearly than most of
them now do in how far children's traits and possibilities are or can be
fixed by inheritance as contrasted with environmental conditions and
nurture in home, school, church and institutional life.
M. V. O'SHEA.
Madison, Wisconsin.
PREFACE
One of the most significant processes at work in society to-day is the
awakening of the civilized world to the rights of the child; and it is
coming to be realized that its right of rights is that of being well-born.
Any series of publications, therefore, dealing primarily with the problems
of child nature may very fittingly be initiated by a discussion of the
factor of well-nigh supreme importance in determining this nature,
heredity.
No principles have more direct bearing on the welfare of man than those of
heredity, and yet on scarcely any subject does as wide-spread ignorance
prevail. This is due in part to the complexity of the subject, but more to
the fact that in the past no clear-cut methods of attacking the manifold
problems involved had been devised. Happily this difficulty has at least
in part been overcome.
It is no exaggeration to say that during the last fifteen years we have
made more progress in measuring the extent of inheritance and in
determining its elemental factors than in all previous time. Instead of
dealing wholly now with vague general impressions and speculations,
certain definite principles of genetic transmission have been disclosed.
And since it is becoming more and more apparent that these hold for man as
well as for plants and animals in general, we can no longer ignore the
social responsibilities which the new facts thrust upon us.
Since what a child becomes is determined so largely by its inborn
capacities it is of the greatest importance that teachers and parents
realize something of the nature of such aptitudes before they begin to
awaken them. For education consists in large measure in applying the
stimuli necessary to set going these potentialities and of affording
opportunity for their expression. Of the good propensities, some will
require merely the start, others will need to be fostered and coaxed into
permanence through the stereotyping effects of proper habits; of the
dangerous or bad, some must be kept dormant by preventing improper
stimulation, others repressed by the cultivation of inhibitive tendencies,
and yet others smothered or excluded by filling their place with desirable
traits before they themselves come into expression.
We must see clearly, furthermore, that even the best of pedagogy and
parental training has obvious limits. Once grasp the truth that a child's
fate in life is frequently decided long before birth, and that no amount
of food or hospital service or culture or tears will ever wholly make good
the deficiencies of bad "blood," or in the language of the biologist, a
faulty germ-plasm, and the conviction must surely be borne home to the
intelligent members of society that one thing of superlative importance in
life is the making of a wise choice of a marriage mate on the one hand,
and the prevention of parenthood to the obviously unfit on the other.
In the present volume it is intended to examine into the natural
endowment of the child. And since full comprehension of it requires some
understanding of the nature of the physical mechanism by which hereditary
traits are handed on from generation to generation, a small amount of
space is given to this phase. Then, that the reader may appreciate to
their fullest extent the facts gathered concerning man, a review of the
more significant principles of genetics as revealed through experiments in
breeding plants and animals has been undertaken. The main applications of
these principles to man is pointed out in a general discussion of human
heredity. Finally, inasmuch as all available data indicate that the fate
of our very civilization hangs on the issue, the work concludes with an
account of the new science of eugenics which is striving for the
betterment of the race by determining and promulgating the laws of human
inheritance so that mankind may intelligently go about conserving good and
repressing bad human stocks.
In order to eliminate as many errors as possible and to avoid oversights I
have submitted various chapters to certain of my colleagues and friends
who are authorities in the special field treated therein. While these
gentlemen are in no way responsible for the material of any chapter they
have added greatly to the value of the whole by their suggestions and
comments. Thus I am indebted to Professor Leon J. Cole for reading the
entire manuscript; to Professors A. S. Pearse and F. C. Sharp for reading
Chapter VII; to Professor C. R. Bardeen for reading special parts; to
Doctor J. S. Evans for reading Chapter VI and part of V; to Doctor W. F.
Lorenz, of the Mendota Hospital, for reading Chapter VIII; to Judge E. Ray
Stevens for reading Chapter IX, and to Helen M. Guyer for several readings
of the entire manuscript.
Grateful acknowledgment is made to all of these readers, to various
publishers and periodicals for the use of certain of the illustrations, to
the authors of the numerous books and papers from which much of the
material in such a work as this must necessarily be selected, and to my
artist, Miss H. J. Wakeman, for her painstaking endeavors to make her work
conform to my ideas of what each diagram should show.
M. F. G.
CONTENTS
CHAPTER PAGE
I HEREDITY 1
Blood heritage--Kind determined by origin--Ancestry a network--
Ancestry in royalty--Offspring derived from one parent only--Dual
ancestry an aid in studying heredity--Reversion--Telegony--
Prenatal influences apart from heredity--Parent body and germ not
identical--A hereditary character defined--Hereditary mingling a
mosaic rather than a blend--Determiners of characters, not
characters themselves, transmitted--Our knowledge of heredity
derived along three lines--The method of experimental breeding--
The statistical method--Galton's law of regression--Correlations
between parents and offspring--The biometrical method,
statistical, not physiological--Mental as well as physical
qualities inheritable.
II THE BEARERS OF THE HERITAGE 20
The cell the unit of structure--Unicellular organisms--Importance
of cell-theory--Heredity in unicellular forms--Reproduction and
heredity in colonial protozoa--Conjugation--Specialization of
sex-cells--The fertilized ovum--Advancement seen in the Volvox
colony--Natural death--Specialization in higher organisms--Sexual
phenomena in higher forms--Cell-division--Chromosomes constant in
number and appearance--Significance of the chromosomes--Cleavage
of the egg--Chief processes operative in building the body--The
origin of the new germ-cells--Significance of the early setting
apart of the germ-cells--Individuality of chromosomes--Pairs of
chromosomes--Reduction of the number of chromosomes by one-half--
Maturation of the sperm-cell--Maturation of the egg-cell--Parallel
between the two processes--Fertilization--Significance of the
behavior of the chromosomes--A single set of chromosomes
sufficient for the production of an organism--The duality of the
body and the singleness of the germ--The cytoplasm in
inheritance--Chromosomes possibly responsible for the
distinctiveness of given characters--Sex and heredity--Many
theories of sex determination--The sex-chromosome--Sex-linked
characters in man--In lower forms.
III MENDELISM 67
New discoveries in the field of heredity--Mendel--Rediscovery of
Mendelian principles--Independence of inheritable characters--
Illustration in the Andalusian fowl--The cause of the ratio--
Verification of the hypothesis--Dominant and recessive--
Segregation in the next generation--Illustrated in guinea-pigs--
Terminology--The theory of presence and absence--Additional
terminology--Dominance not always complete--Modifications of
dominance--Mendel's own work--Dihybrids--Getting new combinations
of characters--Segregations of the determiners--Four kinds of
gametes in each sex--The 9:3:3:1 ratio--Phenotype and genotype--
The question of blended inheritance--Nilsson-Ehle's discoveries--
Such cases easily mistaken for true blends--Skin-color in man--
Questionable if real blends exist--The place of the Mendelian
factors in the germ-cell--Parallel between the behavior of
Mendelian factors and chromosomes--A single chromosome not
restricted to carrying a single determiner.
IV MENDELISM IN MAN 97
Probably applicable to many characters in man--Difficult to get
correct data--A generalized presence-absence formula--Indications
of incomplete dominance--Why after the first generation only half
the children may show the dominant character--Eye-color in man--
Hair-color--Hair-shape--Irregularities--Digital malformations--Eye
defects--Other defects inherited as dominants--Recessive
conditions more difficult to deal with--Albinism--Other recessive
conditions in man--Breeding out defects--Other inheritable
conditions in man.
V ARE MODIFICATIONS ACQUIRED DIRECTLY BY THE BODY INHERITED? 121
Which new characters are inherited?--Examples of somatic
modifications--Use and disuse--The problem stated--Special
conditions in mammals--Three fundamental questions--External
influences may directly affect the germ-cells--Such effects
improbable in warm-blooded animals--Poisons may affect the
germ-plasm--How can somatic modifications be registered in
germ-cells?--Persistence of Mendelian factors argues against such
a mode of inheritance--Experiments on insects--On plants--On
vertebrates--Epilepsy in guinea-pigs--Effects of mutilations not
inherited--Transplantation of gonads--Effects of body on germ,
general not specific--Certain characters inexplicable as inherited
somatic acquirements--Neuter insects--Origin of new characters in
germinal variation--Sexual reproduction in relation to new
characters--Many features of an organism characterized by
utility--Germinal variation a simpler and more inclusive
explanation--Analysis of cases--Effects of training--Instincts--
Disease--Reappearance not necessarily inheritance--Prenatal
infection not inheritance--Inheritance of a predisposition not
inheritance of a disease--Tuberculosis--Two individuals of
tubercular stock should not marry--Special susceptibility less of
a factor in many diseases--Deaf-mutism--Gout--Nervous and mental
diseases--Other disorders which have hereditary aspects--Induced
immunity not inherited--Social, ethical and educational
significance of non-inheritance of somatic modifications--No cause
for discouragement--Improved environment will help conserve
superior strains when they do appear.
VI PRENATAL INFLUENCES 151
All that a child possesses at birth not necessarily hereditary--
The myth of maternal impressions--Injurious prenatal influences--
Lead poisoning--The expectant mother should have rest--Too short
intervals between children--Expectant mothers neglected--
Alcoholism--Unreliability of most data--Alcohol a germinal and
fetal poison--Various views of specialists on the effects of
alcoholism on progeny--The affinity of alcohol for germinal
tissue--Innate degeneracy versus the effects of alcohol--
Experimental alcoholism in lower animals--Further remarks on the
situation in man--Much inebriety in man due to defective nervous
constitution--Factors to be reckoned with in the study of
alcoholism--Venereal diseases--The seriousness of the situation--
Infantile blindness--Syphilis--Some of the effects--A blood test--
Many syphilitics married--Why permit existing conditions to
continue?--Ante-nuptial medical inspection--The perils of venereal
disease must be prevented at any cost--Bad environment can wreck
good germ-plasm.
VII RESPONSIBILITY FOR CONDUCT 195
All mental process accompanied by neural process--Gradations in
nervous response from lower organisms to man--Behavior of many
animals often an automatic adjustment to simple external agents--
Tropisms--Certain apparently complex volitions probably only
tropisms--Complicating factors--Many tropic responses apparently
purposeful--Tropisms grade into reflex actions and instincts--
Adjustability of instincts opens the way for intelligent
behavior--Modification of habits possible in lower animals--Some
lower vertebrates profit by experience--Rational behavior--
Conceptual thought probably an outgrowth of simpler psychic
states--The capacity for alternative action in higher animals--The
elemental units of the nervous system are the same in lower and
higher animals--Neuron theory--Establishment of pathways through
the nervous system--Characteristic arrangements of nerve cells
subject to inheritance--Different parts of the cortex yield
different reactions--Skill acquired in one branch of learning
probably not transferred to another branch--Preponderance of
cortex in highest animals--Special fiber tracts in the spinal cord
of man and higher apes--Great complexity in associations and more
neurons in the brain of man--The nervous system in the main
already staged at the time of birth--Many pathways of conduction
not yet established--The extent of the modifiable zone unknown--
Various possibilities of reaction in the child--Probable origin of
altruistic human conduct--Training in motive necessary--Actual
practise in carrying out projects important--Interest and
difficulty both essential--The realization of certain
possibilities of the germ rather than others is subject to
control--We must afford the opportunity and provide the proper
stimuli for the development of good traits--Moral responsibility.
VIII MENTAL AND NERVOUS DEFECTS 228
Prevalence of insanity--Imperfect adjustments of the brain
mechanism inheritable--Many mental defectives married--
Disproportionate increase in number of mental defectives--Protests
voiced by alienists--Examples of hereditary feeble-mindedness--
Difficult to secure accurate data--Feeble-mindedness and insanity
not the same--Many types of insanity--Not all insanities of the
same eugenical significance--Difficulties of getting genealogies
of specific forms of insanity--Certain forms of insanity seem to
behave as Mendelian recessives--Grades of feeble-mindedness--About
two-thirds of feeble-mindedness inherited--Some results of
non-restraint of the feeble-minded--Not all cases of mental
deficiency inherited--Epileptics--Feeble-mindedness probably a
recessive--Many apparently normal people are carriers of
neuropathic defects--Tests for mental deficiency--The backward
child in school--The exceptionally able child--Cost of caring for
our mentally disordered--Importance of rigid segregation of the
feeble-minded--Importance of early diagnosis of insanity--Opinion
of competent psychiatrists essential--Some insanities not
hereditary--Importance of heredity in insanity not appreciated.
IX CRIME AND DELINQUENCY 263
Heredity and environment in this field--Feeble-mindedness often a
factor--Many delinquent girls mentally deficient--Institutional
figures misleading--Many prisoners mentally subnormal--Inhibitions
necessary to social welfare--The high-grade moron a difficult
problem--Degenerate strains--Intensification of defects by
inbreeding--Vicious surroundings not a sufficient explanation in
degenerate stocks--Not all delinquents defectives--No special
inheritable crime-factor--What is a born criminal?--Epileptic
criminal especially dangerous--The mental disorders most
frequently associated with crime--Bearing of immigration on crime
and delinquency--Sexual vice--School instruction in sex-hygiene--
Mere knowledge not the crux of the sex problem--Early training in
self-restraint an important preventive of crime and delinquency--
Multiplication of delinquent defectives must be prevented.
X RACE BETTERMENT THROUGH HEREDITY 289
Questionable charity--Past protests--An increasing flood of
defectives--Natural elimination of defectives done away with--Why
not prevent our social maladies?--Eugenics defined--Improved
environment alone will not cure racial degeneracy--Heredity and
environment--Inter-racial marriage--Human conservation--Kindness
in the long run--The problem has two phases--Constructive eugenics
must be based on education--Inferior increasing more rapidly than
superior stocks--An unselected population may contain much
valuable material--The lack of criteria for judging fitness--The
college graduate--Native ability, independence and energy
eugenically desirable--Four children to each marriage required to
maintain a stock--Factors contributing to low birth-rate in
desirable strains--The educated public must be made to realize the
situation--Utilization of family pride as a basis for constructive
eugenics--The tendency for like to marry like--Public opinion as
an incentive to action--Choosing a marriage mate means choosing a
parent--The best eugenic marriage also a love match--The
elimination of the grossly unfit urgent--Suggested remedies--
Inefficacy of laws which forbid marriage of mental defectives--
Systems of mating impracticable in the main--Corrective mating
presupposes knowledge of eugenics--Segregation has many
advocates--Sterilization as a eugenic measure--To what conditions
applicable--In insanity--In feeble-mindedness--In cases of
epilepsy--Sterilization laws--Social dangers in vasectomy--Our
present knowledge insufficient--Sterilization laws on trial--An
educated public sentiment the most valuable eugenic agent--The
question of personal liberty--Education of women in eugenics
needed--Much yet to be done--A working program--Which shall it be?
GLOSSARY 343
REFERENCES FOR FURTHER READING AND STUDY 355
INDEX 361
BEING WELL-BORN
CHAPTER I
HEREDITY
It is a commonplace fact that offspring tend to resemble their parents. So
commonplace, indeed, that few stop to wonder at it. No one misunderstands
us when we say that such and such a young man is "a chip off the old
block," for that is simply an emphatic way of stating that he resembles
one or the other of his parents. The same is true of such familiar
expressions as "what's bred in the bone," "blood will tell," and kindred
catch phrases. All are but recognitions of the same common fact that
offspring exhibit various characteristics similar to those of their
progenitors.
=Blood Heritage.--=To this phenomenon of resemblance in successive
generations based on ancestry the term heredity is applied. In man, for
instance, there is a marked tendency toward the reappearance in offspring
of structures, habits, features, and even personal mannerisms, minute
physical defects, and intimate mental peculiarities like those possessed
by their parents or more remote forebears. These personal characteristics
based on descent from a common source are what we may call the blood
heritage of the child to discriminate it from a wholly different kind of
inheritance, namely, the passing on from one generation to the next of
such material things as personal property or real estate.
=Kind Determined by Origin.--=It is inheritance in the sense of community
of origin that determines whether a given living creature shall be man,
beast, bird, fish, or what not. A given individual is human because his
ancestors were human. In addition to this stock supply of human qualities
he has certain well-marked features which we recognize as characteristics
of race. That is, if he is of Anglo-Saxon or Italian or Mongolian
parentage, naturally his various qualities will be Anglo-Saxon, Italian,
or Mongolian. Still further, he has many distinctive features of mind and
body that we recognize as family traits and lastly, his personal
characteristics such as designate him to us as Tom, Harry, or James must
be added. The latter would include such minutiae as size and shape of ears,
nose or hands; complexion; perhaps even certain defects; voice; color of
eyes; and a thousand other particulars. Although we designate these
manifold items as individual, they are in reality largely more or less
duplicates of similar features that occur in one or the other of his
progenitors, features which he would not have in their existing form but
for the hereditary relation between him and them.
"O Damsel Dorothy! Dorothy Q.!
Strange is the gift that I owe to you;
* * * * *
What if a hundred years ago
Those close-shut lips had answered 'No,'
* * * * *
Should I be I, or would it be
One-tenth another, to nine-tenths me?"
"Soft is the breath of a maiden's yes;
Not the light gossamer stirs with less;
But never a cable that holds so fast
Through all the battles of wave and blast,
And never an echo of speech or song
That lives in the babbling air so long!
There were tones in the voice that whispered then
You may hear to-day in a hundred men."
When life steps into the world of matter there comes with it a sort of
physical immortality, so to speak; not of the individual, it is true, but
of the race. But the important thing to note is that the race is made up,
not of a succession of wholly unrelated forms, but a continuation of the
same kind of living organisms, and this sameness is due to the actual
physical descent of each new individual from a predecessor. In other
words, any living organism is the kind of organism it is in virtue of its
hereditary relation to its ancestors.
It is part of the biologist's task to seek a material basis, a continuity
of actual substance, for this continuity of life and form between an
organism and its offspring. Moreover, inasmuch as the offspring is never
precisely similar to its progenitors he must determine also what qualities
are susceptible of transmission and in what measure.
=Ancestry a Network.--=From the fact that each child has all of the
ancestors of its mother as well as of its father, arises the great
complications which are met with in determining the lineage of an
individual. A person has two parents, four grandparents, eight great
grandparents, and thus following out pedigree it is plain to be seen that
through this process of doubling in each generation, in the course of a
few centuries one's ancestry is apparently enormous. By actual
computation, according to Professor D. S. Jordan, if we count thirty
generations back to the Norman invasion of England in 1066, at this ratio
of duplication, the child of to-day would have had at that time an
ancestry of 8,598,094,592 persons. But we know that the total number of
inhabitants in England during the time of William the Conqueror was but a
small fraction of this enormous aggregate. This means that we shall have
to modify our inference that a child has twice as many ancestors as its
parents; a condition which at first sight seems evident, but which is not
literally true. The fact is that the parents of the child, in all
probability, have many ancestors in common--a state of affairs which is
brought about through the intermarriage of relatives, and this is
especially frequent among remoter descendants of common progenitors. Time
after time in genealogy strains of blood have crossed and recrossed until
it is not improbable that a man of to-day who is of English origin has the
blood in his veins from every inhabitant of England who lived during the
time of William the Conqueror and left fruitful descendants. Instead of
conceiving of ancestry as an ever branching and widening tree-like system
as it recedes into the past, it is more accurate, therefore, to regard it
in the light of an elaborate meshwork. The "family tree" in reality
becomes the family net.
=Ancestry in Royalty.--=The pedigrees of royal families have proved to be
of much importance in the study of human inheritance, not that royal
traits are any more heritable than any other, but simply because the
records have been carefully kept so that they are the most comprehensive
and easily followed pedigrees available. The netlike weave of ancestry is
particularly well exemplified in some of these families because of much
close intermarriage. Their heritage typifies on an intensified scale the
heritage of the mass of mankind. For example, if we go six generations
back in the ancestry of Frederick the Great instead of the expected
sixty-four individual ancestors we find only forty; or in a still more
closely woven stock, in the Spanish royal line of Don Carlos we find in
six generations instead of sixty-four individual ancestors, only
twenty-eight. While the present German emperor might have had four
thousand ninety-six ancestors in the twelfth generation back, it is
estimated that owing to intermarriage he probably had only five hundred
thirty-three.
=Offspring Derived from One Parent Only.--=So far in our reckoning of
heredity we have counted elements from both father and mother, and the
complications which arise from such a double ancestry are manifestly very
perplexing ones. If we could do away with the elements of sex and find
offspring that are derived from one parent only, it would seemingly
simplify our problem very much for we should thus have a direct line of
descent, free from intermingling. This, in fact, occurs to a greater or
less extent among lower animals in a number of instances. There may be
only female forms for a number of generations and the eggs which they
produce develop directly into new individuals. Moreover, many of the
simpler organisms have the power of dividing their bodies into two and
thus giving rise to two new forms, each of which resembles the parent.
This shows plainly that we may have inheritance without the appearance of
any male ancestor at all, hence sex is not always a necessary factor in
reproduction or heredity. The development of eggs asexually, that is,
without uniting first with a male cognate, is termed _parthenogenesis_.
The ordinary plant louse or aphid which is frequently found upon geraniums
is a familiar example of an animal which reproduces largely in this way.
During the summer only the females exist and they are so astonishingly
fertile that one such aphid and her progeny, supposing none dies, will
produce one hundred million in the course of five generations. In the last
broods of the fall, males and females appear and fertile eggs are produced
which lie dormant through the winter to start the cycle of the next year.
Again, the eggs of some kinds of animals which normally have to unite with
a male germ before they develop, can be made to develop by merely treating
them with chemical solutions. The difference between an offspring derived
in such a manner, and one which has developed from an egg fertilized by
the male is that it is made up of characteristics from only one source,
the maternal.
=Dual Ancestry an Aid in Studying Heredity.--=Although we have the factors
of heredity in a more simplified form in the case of asexual
transmission, as a matter of fact most of our insight into the problems
of heredity has been attained from a study of sexually reproducing forms,
because the very existence of two sets of more or less parallel features
offers a kind of checking up system by which we can follow a given
characteristic.
=Reversion.--=Occasionally, however, plants and animals do not develop the
complete individuality we might expect, but stop short at or re-attain
some ancestral stage along the line of descent, and thus come to resemble
some progenitor perhaps many generations back of their own time. Thus it
is well known that as regards one or more characteristics a child may
resemble a grandparent or often some remote ancestor much more closely
than it does its immediate parent. The reappearance of such ancestral
traits the student of heredity designates as _Reversion_ or _Atavism_.
Reversion may occur apparently in any class of plants or animals. It is
especially pronounced among domesticated forms, which through man's
selection have been produced under more or less artificial conditions. For
example, among fancy breeds of pigeons, there may be an occasional return
to the old slaty blue color of the ancestral rock-pigeon, with two dark
cross-bars on the wings, from which all modern breeds have been derived.
This is almost sure to happen if the fancy varieties are inter-crossed for
two or three generations. Another example of reversion frequently cited is
the occasional reappearance in domestic poultry of the reddish or brownish
color pattern of the ancestral jungle-fowl to which, among modern forms,
the Indian game seems most nearly related in color. Still another example
is the cross-bars or stripes occasionally to be seen on the forelegs of
colts, particularly mules, reminiscent of the extinct wild progenitors
which were supposedly striped.
Fig. 1, p. 9, is a picture of a hybrid between the common fowl and the
guinea-fowl. The chevron-like markings on certain feathers show a
reversion to a type of color pattern that is prevalent among both the
primitive pheasants (the domestic chicken is a pheasant) and the primitive
guinea-fowls. Although the common spotted guinea-fowl may be crossed with
a black chicken which shows no trace of barring, nevertheless the hybrid
offspring are likely to bear a chevron-like pattern such as that shown in
the picture.
There has been much quibbling over the relative meanings of reversion and
atavism. The general idea, whichever term we use, is that there is a
"throwing back" in a noticeable degree through inheritance to some
ancestral condition beyond the immediate parents. A few recent authors
have taken the term atavism in a restricted sense and use it to signify
specifically those not uncommon cases in which a particular character of
an offspring resembles the corresponding character of a grandparent
instead of a parent. Such, for example, as the blue eye-color of a child
with brown-eyed parents, each of whom in turn has had a blue-eyed parent.
The tendency of other authors is to abandon the term entirely because of
the diversity of meaning that has been attached to it in the past.
[Illustration: FIG. 1
Hybrid between the guinea-fowl and the common fowl, showing in many
feathers reversion to a primitive chevron-like barring.]
Certain classes of so-called reversions, such as the case of the eye-color
just cited, are readily explicable on Mendelian principles as we shall see
in a later chapter, but probably not all kinds of phenomena described as
reversion can be so explained. For example, some seem to be cases of
suppressed development. The word reversion, indeed, must be looked on as
a convenient descriptive term rather than as the name of a single specific
condition.
=Telegony.--=There is yet a wide-spread belief in the supposed influence
of an earlier sire on offspring born by the same mother to a later and
different sire. This alleged phenomenon is termed _telegony_. For example,
many dog-breeders assert that if a thoroughbred bitch has ever had pups by
a mongrel father, her later offspring, although sired by a thoroughbred,
will show taints of the former mongrel mating. In such cases the female is
believed to be ruined for breeding purposes. Other supposed instances of
such influences have been cited among horses, cattle, sheep, pigs, cats,
birds, pets of various kinds and even men. The historic case most
frequently quoted is that of Lord Morton's mare which bore a hybrid colt
when bred to a quagga, a striped zebra-like animal now extinct. In later
years the same mare bore two colts, sired by a black Arabian horse. Both
colts showed stripes on the neck and other parts of the body, particularly
on the legs. It was inferred that this striping was a sort of after effect
of the earlier breeding with the quagga. In recent times, however,
Professor Ewart has repeated the experiment a number of times with
different mares using a Burchell zebra as the test sire. Although his
experiments have been devised so as to conduce in every way possible to
telegony his results have been negative. Moreover, it has been pointed out
that the stripes on the legs of the two foals alleged to show telegony
could not have been derived from the quagga sire for, unlike zebras,
quaggas did not have their legs striped. Furthermore it is known that the
occurrence of dark brown stripes on the neck, withers and legs of ordinary
colts is not uncommon, some cases of which have exhibited more zebra-like
markings than those of the colts from Lord Morton's mare. It seems much
more probable, therefore, that the alleged instances are merely cases of
ordinary reversion to the striped ancestral color pattern which probably
characterized the wild progenitors of the domesticated horse.
Various experiments on guinea-pigs, horses, mice and other forms,
especially devised to test out this alleged after-influence of an earlier
sire, have all proved negative and the general belief of the biologist
to-day is that telegony is a myth.
=Prenatal Influences Apart from Heredity.--=In discussing the problems of
heredity it is necessary to consider also the possibilities of external
influences apart from lineage which may affect offspring through either
parent. Although modifications derived directly by the parent, and
prenatal influences in general, are of extremely doubtful value as of
permanent inheritable significance, nevertheless they must be reckoned
with in any inventory of a child's endowment at birth. Impaired vitality
on the part of the mother, bad nutrition and physical vicissitudes of
various kinds all enter as factors in the birthright of the child, who,
moreover, may bear in its veins slumbering poisons from some progenitor
who has handed on blood taints not properly attributable to heredity. Of
such importance is this kind of influence to the welfare of the immediate
child that it will be necessary to discuss it in considerable detail in a
later chapter.
=Parent Body and Germ Not Identical.--=Inasmuch as each new individual
appears to arise from material derived from its parent, taking the
evidence at its face value one might suppose that any peculiarity of
organization called forth in the living substance of the parent would
naturally be repeated in the offspring, but a closer study of the
developing organism from its first inception to maturity shows this to be
probably a wrong conclusion. The parent-body and the reproductive
substance contained in that body are by no means identical. It becomes an
important question to decide, in fact, how much effect, if any, either
permanent or temporary, the parent-body really has on the germ.
A given fertile germ (Fig. 2, p. 13) gives rise by a succession of
divisions to a body which we call the individual, but such a germ also
gives rise to a series of new germ-cells which reside in that individual,
and it is these germ-cells, not something derived from the body, that pass
on the determiners of distinguishing features or qualities from generation
to generation. It is only by grasping the significance of this fact that
we can understand how in certain cases a totally different set of
characters may appear in an offspring than those manifested in either
parent.
=An Hereditary Character Defined.--=By a _character_, in discussions in
heredity, is meant simply a trait, feature or other characteristic of an
organism. Where we can pick out a single definable characteristic which
acts as a unit in heredity, for greater accuracy we term it a
_unit-character_. Many traits are known to be inherited on a unit basis or
are capable of being analyzed into factors which are so inherited. These
unit-characters are in large measure inherited independently of one
another apparently, although cases of characters inherited as a unit along
with other characters are known.
=Hereditary Mingling a Mosaic Rather Than a Blend.--=The independence of
unit-characters in inheritance leads us to the important conclusion that
the mingling of two lines of ancestry into a new individual is in no sense
bringing them into the "melting pot," as it has been picturesquely
expressed, but it is rather to be regarded as the mingling of two mosaics,
each particle of which retains its own individuality, and which, even if
overshadowed in a given generation, may nevertheless manifest its
qualities undimmed in later generations when conditions favorable to its
expression transpire.
[Illustration: FIG. 2
Diagram illustrating germinal continuity. Through a series of divisions a
germ-cell gives rise to a body or a soma and to new germ-cells. The
latter, not the body, give rise in turn to the next generation.]
=Determiners of Characters, Not Characters Themselves, Transmitted.--=The
fact should be thoroughly understood that the actual thing which is
transmitted by means of the germ in inheritance is not the character
itself, but something which will _determine_ the character in the
offspring. It is important to remember this, for often these
_determiners_, as they are called, may lie unexpressed for one or more
generations and may become manifest only in later descendants. The truth
of the matter is, the child does not inherit its characters from
corresponding characters in the parent-body, but parent and child are
alike because they are both products of the same line of germ-plasm, both
are chips from the same old block.
METHODS OF STUDYING HEREDITY
Before entering into details it will be well to get some idea of the
methods which are commonly employed in arriving at conclusions in the
field of heredity. Some of these are extremely complex and all that we can
do in an elementary presentation is to get a glimpse of the procedures.
=Our Knowledge of Heredity Derived Along Three Lines.--=Our modern
conceptions of heredity have been derived mainly from three distinct lines
of investigation: First, from the study of embryology, in which the
biologist concerns himself with the genesis of the various parts of the
individual, and the mechanism of the germs which convey the actual
materials from which these parts spring; second, through experimental
breeding of plants and animals to compare particular traits or features in
successive generations; and third, through the statistical treatment of
observations or measurements of a large number of parents and their
offspring with reference to a given characteristic in order to determine
the average extent of resemblance between parents and children in that
particular respect.
=The Method of Experimental Breeding.--=A tremendous impetus was given to
the method of experimental breeding when it was realized that we can
itemize many of the parts or traits of an organism into entities which are
inherited independently one of another. Such traits, or as we have already
termed them, unit-characters, may be not only independently heritable but
independently variable as well. The experimental method seeks to isolate
and trace through successive generations the separate factors which
determine the individual unit-characters of the organism. In this attempt
cross-breeding is resorted to. Forms which differ in one or more respects
are mated and the progeny studied. Next these offspring are mated with
others of their own kind or mated back with the respective parent types.
In this way the behavior of a particular character may often be followed
and the germinal constitutions of the individuals concerned can be
formulated with reference to it. Inasmuch as we shall give much
consideration to this method in the chapter on Mendelism we need not
consider it further here.
=The Statistical Method.--=The statistical method seeks to obtain large
bodies of facts and to deal with evidence as it appears through
mathematical analysis of these facts. The attempt of its followers is to
treat quantitatively all biological processes with which it is concerned.
Historically Sir Francis Galton was the first to make any considerable
application of statistical methods to the problems of heredity and
variation. In his attempts to determine the extent of resemblance between
relatives of different degree as regards bodily, mental and temperamental
traits, he devised new methods of statistical analysis which constitute
the basis of modern statistical biology, or _biometry_ as it is termed by
its votaries. Professor Karl Pearson in particular has extended and
perfected the mathematical methods of this field and stands to-day as
perhaps its most representative exponent. The system is in the main based
on the calculus of probability. The methods often are highly specialized,
requiring the use of higher mathematics, and are therefore only at the
command of specially trained workers.
Just as insurance companies can tell us the probable length of human life
in a given social group, since although uncertain in any particular case,
it is reducible in mass to a predictable constant, so the biometrician
with even greater precision because of his improved methods can often,
when a large number of cases are concerned, give us the intensity of
ancestral influence with reference to particular characters.
For example, it is clear that by measuring a large number of adult human
beings one can compute the average height or determine the height which
will fit the greatest number. There will be some individuals below and
some above it, but the greater the divergence from this standard height
the fewer will be the individuals concerned.
Galton compared the heights of 204 normal English parents and their 928
adult offspring. In order to equalize the measurements of men and women he
found he had to multiply each female height by 1.08. Then, to take both
parents into account when comparing height of parents to that of children
he added the height of the father to the proportionately augmented height
of the mother and divided by two, thus securing the height of what he
termed the "mid-parent." He found that the mid-parental heights of his
subjects ranged from 64.5 to 72.5 inches, and that the general _mode_ was
about 68.5 inches. It should be mentioned that the _mode_, in a given
population, represents the group containing the largest number of
individuals of one kind; it may or may not coincide with the average. The
children of all mid-parents having a given height were measured next and
tabulated with reference to these mid-parents. The results of Galton's
measurements may be expressed simply as follows:
-----------------------------------------------------------------
|MODE|
----------------------------------------|----|-------------------
Height of mid-parent| | | | | | | | |
in inches |64.5|65.5|66.5|67.5|68.5|69.5|70.5|71.5|72.5
| | | | | | | | |
Average height of | | | | | | | | |
offspring |65.8|66.7|67.2|67.6|68.3|68.9|69.5|69.9|72.2
-----------------------------------------------------------------
=The Law of Regression.--=It is plain from this table that the offspring
of short mid-parents tend to be under average or modal height though not
so far below as their parents. Likewise children of tall parents tend to
be tall but less tall than their parents. This fact illustrates what is
known as Galton's _law of regression_; namely, that if parents in a given
population diverge a certain amount from the mode of the population as a
whole, their children, while tending to resemble them, will diverge less
from this mode. It is clear that the extent of regression is an inverse
measure of the intensity of inheritance from the immediate parents; if the
deviation of the offspring from the general mode were nearly as great as
that of their parents then the intensity of the inheritance must be high;
if but slight--that is, if the offspring regressed nearly to the
mode--then the intensity of the inheritance must be ranked as low. In the
example in question it must be ranked as relatively high. Computations
show that as regards stature the fraction two-thirds represents
approximately the amount of resemblance between the two generations where
both parents are considered.
=Correlations Between Parents and Offspring.--=In modern researches the
conception of mid-parent and mid-grandparent as utilized by Galton has
been largely abandoned. It has been found more convenient as well as more
accurate to keep the measurements of the two parents separate and to deal
with correlations between fathers and sons, fathers and daughters, mothers
and sons, mothers and daughters, brother and brother, etc. Professor
Pearson and his pupils have found for a number of characters that the
correlation between either parent and children, whether sons or daughters,
is relatively close. The correlation between brother and brother, sister
and sister, and brother and sister, usually ranges a little higher than
the corresponding relation between parents and children.
=The Biometrical Method, Statistical, Not Physiological.--=While biometry
may in certain cases go far toward showing us the average intensity of
the inheritance of certain characters it can not replace the method of
the experimental breeder which deals with particular characters in
individual pedigrees. It must be borne in mind that the biometrical method
is a statistical and not a physiological one and that it is applicable
only when large numbers of individuals are considered in mass. It is most
valuable in cases where we are unable sharply to define single characters,
due probably to the concurrent action of a number of independent causes,
or where experiment is impossible so that we have to depend solely on
numerical data gained by observation.
=Mental Qualities Inheritable.--=Galton showed by this method long ago,
and Pearson and his school have extended and more clearly established the
work, that exceptional mental qualities tend to be inherited. While on the
average the children of exceptional parents tend to be less exceptional
than their parents, still they are far more likely to be exceptional than
are the children of average parents. By this method Professor Pearson has
shown that such mental and temperamental attributes as ability, vivacity,
conscientiousness, temper, popularity, handwriting, etc., are as
essentially determined as are physical features through the hereditary
endowment.
CHAPTER II
THE BEARERS OF THE HERITAGE
Before we can make any detailed analysis of the inheritance of characters
we should have some general idea of the physical structure of animals and
particularly some familiarity with the development of an individual from
the egg, as well as some knowledge of the nature of the germ-cells.
=The Cell the Unit of Structure.--=If we examine one of the higher
animals, as, for example, the horse, the dog, or man, we find that it is
made up of a large number of constituents, such as bones, muscles, nervous
elements, blood and other tissues. Each kind of tissue is composed of a
number of living units, ordinarily microscopic in size, which are known as
cells. A careful examination of various cells reveals that although they
may differ greatly in size, shape and minor details, they all alike
possess certain well-marked characteristics. Each when reduced to its
fundamental form is seen to consist of a small mass of living matter
termed protoplasm in which may usually be distinguished two regions--the
cell-body or _cytoplasm_, and the _nucleus_ (Fig. 3, p. 21). Any cell,
whether it be of the brain, of the liver, or from any organ of an animal
or plant, has this same fundamental structure. In addition, a limiting
membrane or wall of some kind is generally present, although it is not a
necessary constituent of all cells.
[Illustration: FIG. 3
Diagram of a cell showing various parts.]
=Unicellular Organisms.--=While such a structure as a tree or a horse is
composed of countless millions of cells, on the other hand numerous
organisms, both plant and animal, exist which consist of only one cell.
Yet this cell is just as characteristically a cell as are the components
of a complex animal or plant. It has the necessary parts, the cell body
and the nucleus. Moreover it exhibits all of the fundamental activities of
life, though in a simplified form, that a complex higher organism does.
=Importance of Cell-Theory.--=This discovery that every living thing is a
single cell or an aggregation of cooperating cells and cell-products is
one of our most important biological generalizations because it has
brought such a wide range of phenomena under a common point of view. In
the first place, the structure of both plants and animals is reducible to
a common fundamental unit of organization. Moreover, both physiological
and pathological phenomena are more readily understood since we recognize
that the functions of the body in health or disease are in large measure
the result of the activities of the individual cells of the functioning
part. Then again, the problems of embryological development have become
much more sharply defined since it could be shown that the egg is a single
cell and that it is through a series of divisions of this cell and
subsequent changes in the new cells thus formed that the new organism is
built up. And lastly, the problem of hereditary transmission has been
rendered more definite and approachable by the discovery that the male
germ is likewise a single cell, that fertilization of the egg is therefore
the union of two cells, and that in consequence the mechanism of
inheritance must be stowed away somehow in these two cells.
=Heredity in Unicellular Forms.--=In unicellular animals one can readily
see how it is possible for an individual always to give rise to its own
kind. One of the simplest of the single-celled animals is the _Ameba_
(Fig. 4, p. 24).
The ameba eats and grows as do other animals. Sooner or later it reaches a
size beyond which it can not increase advantageously, yet it is
continuously taking in new food material which stimulates it to further
growth. Here then is a problem. The ameba solves this difficulty by
dividing to form two amebae. Such a division is illustrated in Fig. 4, p.
24. First the nucleus divides, then the cell-body. When the two new amebae
separate completely each renews the occupation of eating and growing. But
what has become of the parent? Here, where once existed a large adult
ameba are two young amebae. The parent individual as such has disappeared,
yet there has been no death, for we have simply two bits of living jelly
in place of one. They will in turn repeat the same process, so will their
offspring, and thus, barring accident, this growth and reproduction, or
overgrowth as we may regard it, may go on forever, as far as we know. Here
the problem of heredity, or the resemblance of offspring to parent, is not
a very complicated one. The substance of the cell-body and cell-nucleus
divides into two similar halves, so that each descendant has the substance
of the parent in its own body, only it has but half as much. It differs
from the parent, not in quality or kind, but in size.
[Illustration: FIG. 4
Six successive stages in the division of _Ameba polypodia_ (after
Schulze). The nucleus is seen as a dark spot in the interior.]
=Reproduction and Heredity in Colonial Protozoa.--=There are enormous
numbers of these single-celled animals existing in all parts of the world.
Some are simple like the ameba, others are very complex in structure.
Many, after division, move apart and pursue wholly independent courses of
existence. On the other hand we find a modification appearing in some
which is of the greatest importance. After division instead of moving
apart the two cells may remain side by side and divide further to form
two more, these in turn may divide and thus the process goes on until
there is formed what is known as a colony. Each cell of such a colony
resembles the original ancestral cell because each is a part of the actual
substance of that cell. As in the ameba, the first two cells are the
ancestral cell done up in two separate packets, and thus finally the full
quota of cells must be so many separate packets of the same kind of
material. Inasmuch as each is but a repetition of its original ancestor,
it can, and at times does, produce a colony of the same kind as that
ancestor produced.
=Conjugation.--=At longer or shorter intervals, however, we find that two
individuals, on the disruption of the old colony, instead of continuing
the routine of establishing new colonies through a series of cell
divisions, very radically alter their behavior. They unite and fuse into a
single larger individual. This process is called _conjugation_. We find it
occurring even in some species of ameba. The conjugating cells in some
colonies are alike in size and appearance, in others different.
=Specialization of Sex-Cells.--=A beautiful sphere-shaped colony known as
_Volvox_ is to be found occasionally in roadside pools. Depending on the
species of _Volvox_ to which it belongs, the colony may be made up of from
a few hundred to several thousand individuals arranged in a single layer
about the fluid-filled center of the sphere and bound together by a clear
jelly-like inter-cellular substance. Each individual cell also connects
with its neighbors by means of thin threads of living matter. One of the
largest species is _Volvox globator_, one edge of which is represented in
Fig. 5, p. 27. Mutual pressure of the cells gives them a polygonal shape
when viewed from the surface. Each cell, with a few exceptions to be noted
immediately, bears two long flagella, whip-like structures which project
out into the water. The lashing of these flagella gives the ball a rotary
motion and thus it moves about. When the colony has reached its adult
condition and is ready to reproduce itself, certain cells without flagella
and somewhat larger than the ordinary cells become more rounded in outline
and increase considerably in size through the acquisition of food
materials. They are then known as egg cells or ova. Each ovum finally
enters on a series of cell-divisions forming a mass of smaller and smaller
cells which gradually assumes the form of a hollow sphere like the parent
colony. The young colonies thus formed drop into the interior of the
parent colony to escape later to the outside as independent swimming
organisms when the old colony dies and disintegrates.
=The Fertilized Ovum Termed a Zygote.--=After a number of generations of
such asexual reproduction, sexual reproduction occurs. The ova arise as
usual. Certain members of the colony, on the other hand, go to the other
extreme and divide up into bundles of from sixty-four to one hundred
twenty-eight minute slender cells, each provided with flagella for
locomotion. When mature these small flagellate cells, now known as
_spermatozoa_, escape into the interior of the parent colony and swim
about actively. Ultimately each ovum is penetrated by a spermatozoon, the
two cells fuse completely and thus form the single _fertilized ovum_ or
_zygote_. The body-cells of the mother colony finally disintegrate. After
a period of rest each zygote, through a series of cell-divisions, develops
into an adult Volvox. In some species of Volvox a still further advance is
seen, in that instead of both kinds of gametes being produced in the same
colony, the ova may be produced by one colony and the spermatozoa by
another. Here, then, we have the foreshadowings of two sexes as separate
individuals, a phenomenon of universal occurrence among the highest forms
of animal life.
[Illustration: FIG. 5
_Volvox globator_ (from Hegner after Oltmanns). Half of a sexually
reproducing colony: _o_, eggs; _s_, spermatozoa.]
=Advancement Seen in the Volvox Colony.--=In the Volvox colony there is a
distinct advance over the conditions met with in various lower protozoan
colonies in that only certain individuals of the colony take part in the
process of reproduction and these individuals are of two distinct types;
one is a larger, food-laden cell or egg and the other a small, active,
fertilizing cell. The motile forms are produced in much greater numbers
than the eggs, plainly because they have to seek the egg and many will
doubtless perish before this can be accomplished. This disparity in number
is only a means of insuring fertilization of the egg. The remaining cells
of the body carry on the ordinary activities of the colony such as
locomotion and nutrition and have ceased to take any part in the
production of new colonies.
=Natural Death Appears With the Establishment of a Body Distinct from the
Germ.--=Volvox is an organism of unusual interest because in it we see a
prophecy of what is to come. Although still regarded as a colony of
single-celled individuals, it represents in reality a transition between
the whole group of unicellular animals termed protozoa and the many celled
animals characterized by the possession of distinct tissues, known as
_Metazoa_. Moreover, it shows an interesting stage in the establishment of
a body or _soma_ distinct from special reproductive cells which have taken
on the function of reproducing the colony. In such colonial forms natural
death is found appearing for the first time, the reproductive cells alone
continuing to perpetuate the species. Then again Volvox represents an
important step in the establishment of sex in the animal kingdom for in
its sexual reproduction the conjugating cells known as _gametes_ are no
longer alike in appearance but have become differentiated into definite
ova and spermatozoa.
In Volvox as in the other organisms which we have studied we find that all
of the cells including the germ-cells are produced by the repeated
division of a parent cell, and consequently each must contain the
characteristic living substance of that parent. Many other forms might be
cited to illustrate reproduction in single-celled animals, whether free or
in colonies, but all such cases would be practically but repetitions or
modifications of those we have already examined.
=Specialization in Higher Organisms.--=If we pass on to the higher animals
and plants which are not single cells or colonies of similar cells but
organisms made up of many different kinds of cells, we find a pronounced
extension of the phenomenon met with in Volvox. Instead of each cell
executing independently all of the life relations, certain ones are set
apart for the performance of certain functions to the exclusion of other
functions which are carried on by other members of the aggregation. Thus
the organism as a whole has all the life relations carried on, but, as it
were, by specialists.
=Sexual Phenomena in Higher Forms.--=In the reproduction of multicellular
organisms, one sees likewise but a continuation of the phenomena exhibited
in Volvox. Ordinarily, each new form is produced by the successive
divisions of a single germ-cell which in the vast majority of cases has
conjugated with another germ-cell. In the development of the egg, as the
divisions proceed, groups of cells become modified for their particular
work until the entire organism is completed. During development certain
cells are set apart for reproduction of the form just as they were in
Volvox. These two kinds of reproductive cells in multicellular organisms
are derived ordinarily from two separate individuals known as male and
female, though there are some exceptions. The main difference between
these cells which will have to unite to form a single fertile germ-cell,
is that they have specialized in different directions; one is small and
active, the other large, food-laden and passive. But with two such
germ-cells coming as they do from two individuals, one the male, the other
the female, it is obvious that the actual living substance of which each
germ is composed will be distinctive of its own parental line and that
when the germs unite these distinctive factors commingle, hence the
complications of double ancestry arise.
=Structure of the Cell.--=Before we can understand certain necessary
details of the physical mechanism of inheritance we must inquire a little
further into the finer structure of the cell and into the nature of cell
division. A typical cell, as it would appear after treatment with various
stains which bring out the different parts more distinctly, is shown in
Fig. 3, p. 21. Typical, not that any particular kind of living cell
resembles it very closely in appearance, but because it shows in a
diagrammatic way the essential parts of a cell. In the diagram, there are
two well-marked regions; a central _nucleus_ and a peripheral cell-body or
_cytoplasm_. Other structures are pictured but only a few of them need
command our attention at present. At one side of the nucleus one observes
a small dot or granule surrounded by a denser area of cytoplasm. This body
is called the _centrosome_. The nucleus in this instance is bounded by a
well-marked nuclear membrane and within it are several substances. What
appear to be threads of a faintly staining material, the _linin_, traverse
it in every direction and form an apparent network. The parts on which we
wish particularly to rivet our attention are the densely stained
substances scattered along or embedded in the strands of this network in
irregular granules and patches. This substance is called _chromatin_. It
takes its name from the fact that it shows great affinity for certain
stains and becomes intensely colored by them. This deeply colored portion
of the cell, the chromatin, is by most biologists regarded as of great
importance from the standpoint of heredity. One or more larger masses of
chromatin or chromatin-like material, known as _chromatin nucleoli_, are
often present, and not infrequently a small spheroidal body, differing in
its staining reactions from the chromatin-nucleolus and sometimes called
the _true nucleolus_, exists.
=Cell-Division.--=In the simplest type of cell-division the nucleus first
constricts in the middle, and finally the two halves separate. This
separation is followed by a similar constriction and final division of the
entire cell-body, which results in the production of two new cells. This
form of cell-division is known as _simple_ or _direct division_. Such a
simple division, while found in higher animals, is less frequent and
apparently much less significant than another type of division which
involves profound changes and rearrangements of the nuclear contents. The
latter is termed _mitotic_ or _indirect_ cell-division. Fig. 6, p. 33,
illustrates some of the stages which are passed through in indirect
cell-division. The centrosome which lies passively at the side of the
nucleus in the typical cell (Fig. 6_a_, p. 33) awakens to activity,
divides and the two components come to lie at the ends of a fibrous
spindle. In the meantime, the interior of the nucleus is undergoing a
transformation. The granules and patches of chromatin begin to flow
together along the nuclear network and become more and more crowded until
they take on the appearance of one or more long deeply-stained threads
wound back and forth in a loose skein in the nucleus (Fig. 6_b_, p. 33).
If we examine this thread closely, in some forms it may be seen to consist
of a series of deeply-stained chromatin granules packed closely together
intermingled with the substance of the original nuclear network.
As the preparations for division go on the coil in the nucleus breaks up
into a number of segments which are designated as _chromosomes_ (Fig.
6_c_, p. 33). The nuclear membrane disappears. The chromosomes and the
spindle-fibers ultimately become related in such a way that the
chromosomes come to lie at the equator of the spindle as shown in Fig
6_d_, p. 33. Each chromosome splits lengthwise to form two daughter
chromosomes which then diverge to pass to the poles of the spindle (Figs.
6_e_ and _f_, p. 33). Thus each end of the spindle comes ultimately to be
occupied by a set of chromosomes. Moreover each set is a duplicate of the
other, because the substance of any individual chromosome in one group has
its counterpart in the other. In fact this whole complicated system of
indirect division is regarded by most biologists as a mechanism for
bringing about the precise halving of the chromosomes.
[Illustration: FIG. 6
Diagram showing representative stages in mitotic or indirect
cell-division: _a_, resting cell with reticular nucleus and single
centrosome; _b_, the two new centrosomes formed by division of the old one
are separating and the nucleus is in the spireme stage; _c_, the nuclear
wall has disappeared, the spireme has broken up into six separate
chromosomes, and the spindle is forming between the two centrosomes; _d_,
equatorial plate stage in which the chromosomes occupy the equator of the
spindle; _e_, _f_, each chromosome splits lengthwise and the daughter
chromosomes thus formed approach their respective poles; _g_,
reconstruction of the new nuclei and division of the cell body; _h_,
cell-division completed.]
The chromosomes of each group at the poles finally fuse and two new
nuclei, each similar to the original one, are constructed (Figs. 6_g_ and
_h_, p. 33). In the meantime a division of the cell-body is in progress
which, when completed, results in the formation of two complete new cells.
As all living matter if given suitable food, can convert it into living
matter of its own kind, there is no difficulty in conceiving how the new
cell or the chromatin material finally attains to the same bulk that was
characteristic of the parent cell. In the case of the chromatin, indeed,
it seems that there is at times a precocious doubling of the ordinary
amount of material before the actual division occurs.
=Chromosomes Constant in Number and Appearance.--=With some minor
exceptions, to be noted later, which increase rather than detract from the
significance of the facts, the chromosomes are always the same in number
and appearance in all individuals of a given species of plants or animals.
That is, every species has a fixed number which regularly recurs in all of
its cell-divisions. Thus the ordinary cells of the rat, when preparing to
divide, each display sixteen chromosomes, the frog or the mouse,
twenty-four, the lily twenty-four, and the maw-worm of the horse only
four. The chromosomes of different kinds of animals or plants may differ
very much in appearance. In some they are spherical, in others rod-like,
filamentous or perhaps of other forms. In some organisms the chromosomes
of the same nucleus may differ from one another in size, shape and
proportions, but if such differences appear at one division they appear at
others, thus showing that in such cases the differences are constant from
one generation to the next.
=Significance of the Chromosomes.--=The question naturally arises as to
what is the significance of the chromosomes. Why is the accurate
adjustment which we have noted for their division necessary? The very
existence of an elaborate mechanism so admirably adapted to their precise
halving, predisposes one toward the belief that the chromosomes have an
important function which necessitates the retention of their individuality
and their equal division. Many biologists accept this along with other
evidence as indicating that in chromatin we have a substance which is not
the same throughout, that different regions of the same chromosome have
different physiological values.
When the cell prepares for divisions, the granules, as we have seen,
arrange themselves serially into a definite number of strands which we
have termed chromosomes. Judging from all available evidence, the granules
are self-propagating units; that is, they can grow and reproduce
themselves. So that what really happens in mitosis in the splitting of the
chromosomes is a precise halving of the series of individual granules of
which each chromosome is constituted, or in other words each granule has
reproduced itself. Thus each of the two daughter cells presumably gets a
sample of every kind of chromosomal particle, hence, the two cells are
qualitatively alike. To use a homely illustration we may picture the
individual chromosomes to ourselves as so many separate trains of freight
cars, each car of which is loaded with different merchandise. Now, if
every one of the trains could split along its entire length and the
resulting halves each grow into a train similar to the original, so that
instead of one there would exist two identical trains, we should have a
phenomenon analogous to that of a dividing chromosome.
=Cleavage of the Egg.--=It is through a series of such divisions as these
that the zygote or fertilized egg-cell builds up the tissues and organs of
the new organism. The process is technically spoken of as _cleavage_.
Cleavage generally begins very shortly after fertilization. The fertile
egg-cell divides into two, the resulting cells divide again and thus the
process continues, with an ever-increasing number of cells.
=Chief Processes Operative in Building the Body.--=Although of much
interest, space will not permit of a discussion in detail of the building
up of the special organs and tissues of the body. It must suffice merely
to mention the four chief processes which are operative. These are, (1)
infoldings and outfoldings of the various cell complexes; (2)
multiplication of the component cells; (3) special changes (_histological
differentiation_) in groups of cells; and (4) occasionally resorption of
certain areas of parts.
=The Origin of the New Germ-Cells.--=On account of the unusual importance
from the standpoint of inheritance, which attaches to the germ-cells, a
final word must be said about their origin in the embryo. While the
evidence is conflicting in some cases, in others it has been well
established that the germ-cells are set apart very early from the cells
which are to differentiate into the ordinary body tissues. Fig. 7_A_, p.
38, shows a section through the eight-celled stage of _Miastor_, a fly, in
which a single large, primordial germ-cell (_p. g. c._) has already been
set apart at one end of the developing embryo. The nuclei of the rest of
the embryo still lie in a continuous protoplasmic mass which has not yet
divided up into separate cells. The densely stained nuclei at the opposite
end of the section are the remnants of nurse-cells which originally
nourished the egg. Fig. 7_B_, p. 38, is a longitudinal section through a
later stage in the development of _Miastor_; the primitive germ-cells
(ooeg) are plainly visible. Still other striking examples might be cited.
Even in vertebrates the germ-cells may often be detected at a very early
period.
=Significance of the Early Setting Apart of the Germ-Cells.--=It is of
great importance for the reader to grasp the significance of this early
setting apart of the germ-cells because so much in our future discussion
hinges on this fact. The truth of the statement made in a previous chapter
that the body of an individual and the reproductive substance in that body
are not identical now becomes obvious. For in such cases as those just
cited one sees the germinal substance which is to carry on the race set
aside at an early period in a given individual; it takes no part in the
formation of that individual's body, but remains a slumbering mass of
potentialities which must bide its time to awaken into expression in a
subsequent generation. Thus an egg does not develop into a body which in
turn makes new germ-cells, but body and germ-cells are established at the
same time, the body harboring and nourishing the germ-cells, but not
generating them (Fig. 2, p. 13). The same must be true also in many cases
where the earliest history of the germ-cells can not be visibly followed,
because in any event, in all higher animals, they appear long before the
embryo is mature and must therefore be descendants of the original
egg-cell and not of the functioning tissues of the mature individual. This
need not necessarily mean that the germ-cells have remained wholly
unmodified or that they continue uninfluenced by the conditions which
prevail in the body, especially in the nutritive blood and lymph stream,
although as a matter of fact most biologists are extremely skeptical as to
the probability that influences from the body beyond such general
indefinite effects as might result from under-nutrition or from poisons
carried in the blood, modify the intrinsic nature of the germinal
substances to any measurable extent.
[Illustration: FIG. 7
A--Germ-cell (_p. g. c._) set apart in the eight-celled stage of cleavage
in _Miastor americana_ (after Hegner). The walls of the remaining seven
somatic cells have not yet formed though the resting or the dividing (_M
p_) nuclei may be seen; _c R_, chromatin fragments cast off from the
somatic cells.
B--Section lengthwise of a later embryo of _Miastor_; the primordial
egg-cells (_ooeg__{3}) are conspicuous (after Hegner).]
=Germinal Continuity.--=The germ-cells are collectively termed the
_germinal protoplasm_ and it is obvious that as long as any race continues
to exist, although successive individuals die, some germinal protoplasm is
handed on from generation to generation without interruption. This is
known as the theory of _germinal continuity_. When the organism is ready
to reproduce its kind the germ-cells awaken to activity, usually
undergoing a period of multiplication to form more germ-cells before
finally passing through a process of what is known at _maturation_, which
makes them ready for fertilization. The maturation process proper, which
consists typically of two rapidly succeeding divisions, is preceded by a
marked growth in size of the individual cells.
=Individuality of Chromosomes.--=Before we can understand fully the
significance of the changes which go on during maturation we shall have to
know more about the conditions which prevail among the chromosomes of
cells. As already noted each kind of animal or plant has its own
characteristic number and types of chromosomes when these appear for
division by mitosis. In many organisms the chromosomes are so nearly of
one size as to make it difficult or impossible to be sure of the identity
of each individual chromosome, but on the other hand, there are some
organisms known in which the chromosomes of a single nucleus are not of
the same size and form (Fig. 8, p. 41). These latter cases enable us to
determine some very significant facts. Where such differences of shape and
proportion occur they are constant in each succeeding division so that
similar chromosomes may be identified each time. Moreover, in all ordinary
mitotic divisions where the conditions are accurately known, these
chromosomes of different types are found to be present as pairs of similar
elements; that is, there are two of each form or size.
=Pairs of Similar Chromosomes in the Nucleus Because One Chromosome Comes
from Each Parent.--=When we recall that the original fertilized egg from
which the individual develops is really formed by the union of two
gametes, ovum and spermatozoon, and that each gamete, being a true cell,
must carry its own set of chromosomes, the significance of the pairs of
similar chromosomes becomes evident; one of each kind has probably been
contributed by each gamete. This means that the zygote or fertile ovum
contains double the number of chromosomes possessed by either gamete, and
that, moreover, each tissue-cell of the new individual will contain this
dual number. For, as we have seen, the number of chromosomes is, with
possibly a few exceptions, constant in the tissue-cells and early
germ-cells in successive generations of individuals. For this to be true
it is obvious that in some way the nuclei of the conjugating gametes have
come to contain only half the usual number. Technically the tissue-cells
are said to contain the _diploid_ number of chromosomes, the gametes the
reduced or _haploid_ number.
[Illustration: FIG. 8
A--Chromosomes of the mosquito (_Culex_) after Stevens.
B--Chromosomes of the fruit-fly (_Drosophila_) after Metz.
Both of these forms have an unusually small number of chromosomes.]
=In Maturation the Number of Chromosomes Is Reduced by One-Half.--=This
halving, or as it is known, _reduction_ in the number of chromosomes is
the essential feature of the process of maturation. It is accomplished by
a modification in the mitotic division in which instead of each chromosome
splitting lengthwise, as in ordinary mitosis, the chromosomes unite in
pairs (Fig. 9_b_, p. 42), a process known technically as _synapsis_, and
then apparently one member of each pair passes entire into one new
daughter cell, the other member going to the other daughter cell (Fig.
9_c_, p. 42). In the pairing preliminary to this _reduction division_,
leaving out of account certain special cases to be considered later,
according to the best evidence at our command the union always takes place
between two chromosomes which match each other in size and appearance.
Since one of these is believed to be of maternal and the other of
paternal origin, the ensuing division separates corresponding mates and
insures that each gamete gets one of each kind of chromosome although it
appears to be a matter of mere chance whether or not a given cell gets the
paternal or the maternal representative of that kind.
[Illustration: FIG. 9
Diagram to illustrate spermatogenesis: _a_, showing the diploid number of
chromosomes (six is arbitrarily chosen) as they occur in divisions of
ordinary cells and spermatogonia; _b_, the pairing (synapsis) of
corresponding mates in the primary spermatocyte preparatory to reduction;
_c_, each secondary spermatocyte receives three, the haploid number of
chromosomes; _d_, division of the secondary spermatocytes to form _e_,
spermatids, which transform into _f_, spermatozoa.]
=Maturation of the Sperm-Cell.--=In the maturation of the male gamete the
germ-cell, now known as a _spermatogonium_, increases greatly in size to
become a _primary spermatocyte_. In each primary spermatocyte the pairing
of the chromosomes already alluded to occurs as indicated in Fig. 9_b_, p.
42, where six is taken arbitrarily to indicate the ordinary or _diploid_
number of chromosomes, and three the reduced or _haploid_ number. The
division of the primary spermatocyte gives rise to two _secondary
spermatocytes_ (_c_), the paired chromosomes separating in such a way that
a member of each pair goes to each secondary spermatocyte. Each secondary
spermatocyte (_d_) soon divides again into two _spermatids_ (_e_), but in
this second division the chromosomes each split lengthwise as in an
ordinary division so that there is no further reduction. In some forms the
reduction division occurs in the secondary spermatocytes instead of the
primary. Each spermatid transforms into a mature spermatozoon (_f_). The
spermatozoa of most animals are of linear form, each with a head, a
middle-piece and a long vibratile tail which is used for locomotion. The
head consists for the most part of the transformed nucleus and is
consequently the part which bears the chromosomes.
=Maturation of the Egg-Cell.--=As regards the behavior of the chromosomes
the maturation of the ovum parallels that of the sperm-cell. There are not
so many primordial germ-cells formed and only one out of four of the
ultimate cells becomes a functional egg. As in maturation of the
sperm-cell there is a growth period in which _ooegonia_ enlarge to become
_primary ooecytes_ (Fig. 10_b_, p. 45). In each primary ooecyte as in the
primary spermatocyte the chromosomes pair and two rapidly succeeding
divisions follow in one of which the typical numerical reduction in the
chromosomes occurs. A peculiarity in the maturation of the ovum is that
there is a very unequal division of the cytoplasm in cell division so that
three of the resulting cells usually termed _polar bodies_ are very small
and appear like minute buds on the side of the fourth or egg-cell proper.
The scheme of this formation of the polar bodies is indicated in Fig. 10,
p. 45. In Fig. 10_b_ the chromosomes are seen paired and ready for the
first division; that is, for the formation of the first polar body. Figs.
10_c_, _d_, p. 45, show the giving off of this body. Note that while only
a small proportion of the cytoplasm passes into this tiny cell, its
chromatin content is as great as that of the ovum. A second polar body
(Figs. 10_e_, _f_, p. 45) is formed by the egg, but in this case each
chromosome splits lengthwise, as in ordinary mitosis, and there is no
further numerical reduction. In the meantime, typically, a third polar
body is formed by division of the first. (Stages _e_, _f_, _g_.)
=Parallel Between the Maturation of Sperm- and Egg-Cell.--=This rather
complex procedure of the germ-cells will be rendered more intelligible
through a careful study of Figs. 9 and 10, pp. 42 and 45, and Fig. 11, p.
46, which indicates the parallel conditions in spermotogenesis and
ooegenesis.
[Illustration: FIG. 10
Diagram to illustrate ooegenesis: _a_, showing the diploid number of
chromosomes (six is arbitrarily chosen) as they occur in ordinary cells
and ooegonia; _b_, the pairing of corresponding mates preparatory to
reduction; _c_, _d_, reduction division, giving off of first polar body;
_e_, egg preparing to give off second polar body, first polar body ready
for division; _f_, _g_, second polar body given off, division of first
polar body completed. The egg nucleus, now known as the female pronucleus,
and each body contain the reduced or haploid number of chromosomes.]
The view now generally held regarding the polar bodies is that they are
really abortive eggs. They later disappear, taking no part in
embryo-formation. It can readily be seen how such an unequal division is
advantageous to the large cell, for it receives all of the rich store of
food material that would be distributed among the four cells if all were
of equal size. This increased amount of food is a favorable provision for
the forthcoming offspring whose nourishment is thus more thoroughly
insured.
[Illustration: FIG. 11
Diagram showing the parallel between maturation of the sperm-cell and
maturation of the ovum.]
On the other hand, all of the sperm-cells develop into complete active
forms, which, as aforesaid, usually become very much elongated and develop
a motile organ of some kind. In such cells an accumulation of food to any
large extent would hinder rather than help them, because it would
seriously interfere with their activity.
=Fertilization.--=In fertilization (Fig. 12, p. 48) the spermatozoon
penetrates the wall of the ovum and after undergoing considerable
alteration its nucleus fuses with the nucleus of the egg. In some forms
only the head (nucleus) and middle-piece enter, the tail being cut off by
a so-called fertilization membrane which forms at the surface of the egg
and effectually blocks the entrance of other spermatozoa. Thus normally
only one spermatozoon unites with an egg. In some forms while several may
enter the egg only one becomes functional. As soon as the nucleus of the
spermatozoon, now known as the male _pronucleus_, reaches the interior of
the egg, it enlarges and becomes similar in appearance to the female
_pronucleus_. It swings around in such a way (Fig. 12_b_, p. 48) that the
middle piece, now transformed into a centrosome, lies between it and the
female pronucleus. The two pronuclei (_c_, _d_, _e_), each containing the
reduced number of chromosomes, approach, the centrosome divides, the
nuclear walls disappear, the typical division spindle forms, and the
chromosomes of paternal and maternal origin respectively come to lie side
by side at the equator of the spindle ready for the first division or
cleavage (_f_, _g_). It will be noted that the individual chromosomes do
not intermingle their substance at this time, but that each apparently
retains its own individuality. There is considerable evidence which
indicates that throughout life the chromosomes contributed by the male
parent remain distinct from those of the female parent. Inasmuch as each
germ-cell, after maturation, contains only half the characteristic number
of chromosomes, the original number is restored in fertilization.
[Illustration: FIG. 12
Diagram to illustrate fertilization; [male], male pronucleus; [female],
female pronucleus; observe that the chromosomes of maternal and paternal
origin respectively do not fuse.]
=Significance of the Behavior of the Chromosomes.--=The question confronts
us as to what is the significance of this elaborate system which keeps the
chromosomes of constant size, shape and number; which partitions them so
accurately in ordinary cell-divisions; and which provides for a reduction
of their numbers by half in the germ-cell while yet securing that each
mature gamete gets one of each kind of chromosome. Most biologists look on
these facts as indicating that the chromosomes are specifically concerned
in inheritance.
In the first place it is recognized that as regards the definable
characters which separate individuals of the same species, offspring may
inherit equally from either parent. And it is a very significant fact that
while the ovum and spermatozoon are very unequal in size themselves, the
chromosomes of the two germ-cells are of the same size and number. This
parity in chromosomal contribution points clearly to the means by which an
equal number of character-determiners might be conveyed from each parent.
Moreover it is mainly the nucleus of the sperm-cell in some organisms
which enters the egg, hence the determiners from the male line must exist
wholly or largely somewhere in the nucleus. And the bulk of the nucleus in
the spermatozoon consists of the chromosomes or their products.
=A Single Set of Chromosomes Derived from One Parent Only Is Sufficient
for the Production of a Complete Organism.--=That a single or haploid set
of chromosomes as seen in the gametes is sufficient contribution of
chromatin for the production of a complete organism is proved by the fact
that the unfertilized eggs of various animals (many echinoderms, worms,
mollusks, and even the frog) may be artificially stimulated to development
without uniting at all with a spermatozoon. The resulting individual is
normal in every respect except that instead of the usual diploid number it
has only the single or haploid number of chromosomes. Its inheritance of
course is wholly of maternal origin. The converse experiment in
echinoderms in which a nucleus of male origin (that is, a spermatozoon)
has been introduced into an egg from which the original nucleus has been
removed shows that the single set of chromosomes carried by the male
gamete is also sufficient to cooperate with the egg-cytoplasm in
developing a complete individual.
=The Duality of the Body and the Singleness of the Germ.--=Since every
maternal chromosome in the ordinary cell has an equivalent mate derived
from the male parent, it follows therefore, supposing the chromosomes do
have the significance in inheritance attributed to them, that as regards
the measurable inheritable differences between two individuals, the
ordinary organism produced through the union of the two germ-cells is,
potentially at least, dual in nature. On the other hand through the
process of reduction the gametes are provided with only a single set of
such representatives. This duality of the body and singleness of the
mature germ is one of the most striking facts that come to light in
embryology. How well the facts fit in with the behavior of certain
hereditary characters will be seen later in our discussions of Mendelism.
=The Cytoplasm Not Negligible in Inheritance.--=Just what part is played
by the cytoplasm in inheritance is not clear, but it is probably by no
means a negligible one. The cytoplasm of a given organism is just as
distinctive of the species or of the individual of which it forms a part
as are the chromosomes. It is well established that neither nucleus nor
cytoplasm can fully function or even exist long without the other, and
neither can alone produce the other. They undoubtedly must cooperate in
building up the new individual, and the cytoplasm of the new individual is
predominantly of maternal origin. It is obvious that all of the more
fundamental characters which make up an organism, such, for instance, as
make it an animal of a certain order or family, as a human being or a dog
or a horse, are common to both parents, and there is no way of measuring
how much of this fundamental constitution comes from either parent, since
only closely related forms will interbreed. In some forms, moreover, the
broader fundamental features of embryogeny are already established before
the entrance of the spermatozoon. It is probable therefore that instead of
asserting that the entire quota of characters which go to make up a
complete individual are inherited from each parent equally, we are
justified only in maintaining that this equality is restricted to those
measurable differences which veneer or top off, as it were, the
individual. We may infer that in the development of the new being the
chromosomes of the egg together with those derived from the male work
jointly on or with the other germinal contents which are mostly
cytoplasmic materials of maternal origin.
=The Chromosomes Possibly Responsible for the Distinctiveness of Given
Characters.--=It seems probable that in the establishment of certain basic
features of the organism the cooperation of the cytoplasm with chromatin
of either maternal or paternal origin might accomplish the same end, but
that certain distinctive touches are added or come cumulatively into
expression through influences carried, predominantly at least, in the
chromatin from one as against the other parent. These last distinctive
characters of the plant or animal constitute the individual differences of
such organisms. In this connection it is a significant fact that in young
hybrids between two distinct species the early stages of development,
especially as regards symmetry and regional specifications, are
exclusively or predominantly maternal in character, but the male influence
becomes more and more apparent as development progresses until the final
degree of intermediacy is attained.
From the evidence at hand this much seems sure, that the paternal and
maternal chromosomes respectively carry substances, be they ferments,
nutritive materials or what not, that are instrumental in giving the final
parity of personal characters which we observe to be equally heritable
from either line of ancestry. It is clear that most of the characters of
an adult organism can not be merely the outcome of any unitary substance
of the germ. Each is the product of many cooperating factors and for the
final outcome any one cooperant is probably just as important in its way
as any other. The individual characters which we juggle to and fro in our
breeding experiments seem apexed, as it were, on more fundamental features
of organic chemical constitution, polarity, regional differentiation, and
physiological balance, but since such individual characters parallel so
closely the visible segregations and associations which go on among the
chromosomes of the germ-cells it would seem that they, at least, are
represented in the chromosomes by distinctive cooperants which give the
final touch of specificity to those hereditary characters which can be
shifted about as units of inheritance.
=Sex and Heredity.--=Whatever the origin of fertilization may have been in
the world of life, or whatever its earliest significance, the important
fact remains that to-day it is unquestionably of very great significance
in relation to the phenomena of heredity. For in all higher animals, at
least, offspring may possess some of the characteristics originally
present in either of two lines of ancestry, and this commingling of such
possessions is possible only through sexual reproduction. As has already
been seen, in the pairing of chromosomes previous to reduction, the
corresponding members of a pair always come together so that in the final
segregation each gamete is sure to have one of each kind although whether
a given chromosome of the haploid set is of maternal or paternal origin
seems to be merely a matter of chance. Thus, for instance, if we
arbitrarily represent the chromosomes of a given individual by _ABC_
_abc_, and regard _A_, _B_ and _C_ as of paternal and _a_, _b_ and _c_ as
of maternal origin, then in synapsis only _A_ and _a_ can pair together,
_B_ and _b_ and _C_ and _c_, but each pair operates independently of the
other so that in the ensuing reduction division either member of a pair
may get into a cell with either member of the other pairs. That is, the
line up for division at a given reduction might be any one of the
following, ABC/abc ABc/abC Abc/aBC AbC/aBc. This would yield the following
eight kinds of gametes, _ABC_, _abc_, _ABc_, _abC_, _Abc_, _aBC_, _AbC_,
_aBc_, each bearing one of each kind of chromosome required to cover the
entire field of characters necessary to a complete organism. And since
each sex would be equally likely to have these eight types of gametes and
any one of the eight in one individual might meet any one of the eight of
the other, the possible number of combinations in the production of a new
individual from such germ-cells would be 8x8, or 64. With the larger
numbers of chromosomes which exist in most animals it is readily seen that
the number of possible combinations becomes very great. Thus any
individual of a species with twenty chromosomes--and many animals,
including man, have more--would have ten pairs at the reduction period and
could therefore form (2)^{10}, or 1,024 different gametes in each sex. And
since any one of these in one sex would have an equal chance of meeting
with any one in the opposite sex, the total number of possible different
zygotes that might be produced would be (1,024)^{2}, or 1,048,576. Sex
therefore, through recombinations of ancestral materials, undoubtedly
means, among other things, the production of great diversity in
offspring.
DETERMINATION OF SEX
=Many Theories.--=From earliest times the problem of sex determination has
been one of keen interest, and needless to say hundreds of theories have
been propounded to explain it. Geddes and Thomson say that Drelincourt
recorded two hundred sixty-two so-called theories of sex production and
remark that since his time the number has at least been doubled. The
desirability of controlling sex has naturally appealed strongly to
breeders of domesticated animals.
A study of animals born in litters, or of twins, is enough in itself to
make us skeptical of theories of sex-determination based on nutritional or
external factors. In a litter of puppies, for example, there are usually
both males and females, although in their prenatal existence they have all
been subject to the same nutritional and environmental conditions.
Likewise in ordinary human twins one may be a boy, the other a girl,
whereas if the nutritional condition of the mother were the fact
determining sex, both should be boys or both girls. However, there are
twins known as _identical twins_ who are remarkably alike and who are
always of the same sex. But there is reason to suppose that identical
twins in reality come from the same zygote. Presumably in early
embryogeny, probably at the two-celled stage of cleavage, the two
blastomeres become separated and each gives rise to a complete individual
instead of only the half of one it would have produced had the two
blastomeres remained together. Such twins are monochorial; that is, they
grow inside the same fetal membrane, whereas each ordinary twin has its
own fetal membrane and has obviously originated from a separate ovum. It
has been established experimentally in several kinds of animals that early
cleavage blastomeres when isolated can each develop into a complete
individual. In man, ordinary twins are no more alike than ordinary
brothers and sisters, but identical twins are strikingly similar in
structure, appearance, habits, tastes, and even susceptibility to various
maladies. The fact that they are invariably of the same sex is a strong
reason for believing that sex was already developed in the fertile ovum
and consequently in the resulting blastomeres from that ovum.
The young of the nine-banded armadillo in a given litter are invariably of
the same sex and are closely similar in all features. Newman and Patterson
have shown that all the members of a litter come from the same egg.
Patterson has established the fact that cleavage of the egg takes place in
the usual manner, but later separate centers of development appear in the
early embryonic mass and give rise to the separate young individuals.
Again in certain insects where one egg indirectly gives rise to a chain of
embryos, or to a number of separate larvae, possibly as many as a thousand,
all of the latter are of the same sex. Even in some plants researches have
shown that sex is already determined at the beginning of development.
Then, too, much evidence has come to light recently showing that
sex-characters in certain cases behave as heritable characters and are
independent of external conditions. Lastly there is visible and convincing
evidence obtainable through microscopical observations that sex is
determined by a mechanism in the germ-cells themselves. It is chiefly to
these latter facts that I wish to direct attention for the present.
=The Sex Chromosome.--=The evidence centers about a special chromosome or
chromosome-group commonly designated as the _sex-chromosome_ or
_X-element_, which has been found in various species of animals, including
man. In the males of such animals this chromosome is present in addition
to the regular number of pairs, thus giving rise to an _uneven_ instead of
the conventional even number of chromosomes. This element remains
undivided in one of the maturation divisions of the spermatocytes, in some
forms in the first in others in the second, and passes entire to one pole
of the spindle (Fig. 13, p. 58). This results in the production of two
classes of cells, one containing the _X_-element and one not. The outcome
is that two corresponding classes of spermatozoa are produced. The
phenomena involved are diagrammatically represented in Fig. 13. It has
been clearly demonstrated in several cases that eggs fertilized by
spermatozoa which possess this _X_-element, always become females, those
fertilized by spermatozoa which do not possess it always develop into
males.
[Illustration: FIG. 13
Diagram illustrating the behavior of the _x_-element or sex-chromosome in
the maturation of the sperm-cell. In one of the two maturation divisions
(represented here as in the first) it passes undivided to one pole (_a_,
_b_, _c_), in the other it divides. Since the cell without the _x_-element
also divides the result is that ultimately from the original primary
spermatocyte (_a_) four cells are formed (_f_), two with the _x_-element
and two without it. Half of the spermatozoa therefore will bear an
_x_-element, half will be without it. In _a_ the ordinary chromosomes,
arbitrarily indicated as 10, are supposed to have already paired for
reduction so that the original diploid number in spermatogonia and
body-cells of the male would be 20 plus the _x_-chromosome.]
It has been found, furthermore, that in species in which the males possess
this extra element the females have two of them. That is, if the original
number in the somatic cells of the male were twenty-three, twenty-two
ordinary and one _X_-element, the number in the somatic cells of the
female would be twenty-four, or twenty-two ordinary and two _X_-elements.
It has been found that when the chromosomes of the female pair for the
reduction division, each chromosome uniting with its corresponding fellow,
the two _X_-elements in the female pair in the usual way so that every
egg-cell possesses an _X_-element. Thus every mature egg has an
_X_-element, while only half of the spermatozoa have one. That is, if we
assume twenty-three as the diploid number present originally in the
somatic cells of the male and twenty-four as the number in the female,
then one-half the spermatozoa of the male would contain the haploid number
eleven, and the other half, the number twelve, whereas every mature ovum
would contain twelve. Since there are equal numbers of the spermatozoa
with the _X_-element and without it, and inasmuch as presumably under
ordinary conditions one kind is as likely to fertilize the egg as the
other, then there are equal chances at fertilization of producing a zygote
with two _X_-elements or with but one.
Thus, Spermatozoon + _X_ by Ovum + _X_ = Zygote + _XX_.
Spermatozoon (no _X_) by Ovum + _X_ = Zygote + _X_.
We have already seen that the former is always female, the latter male. It
thus becomes possible to distinguish the sex of an embryo by counting the
chromosomes of its cells. This has been accomplished in several cases.
In some instances[1] the conditions may be much more complex than the
ones indicated--too complex in fact to warrant detailed discussion in an
elementary exposition--but the principle remains the same throughout, the
very complexity when thoroughly understood, strengthening rather than
weakening the evidence. In a few forms an interesting reversal of
conditions has been found in that the eggs instead of the spermatozoa show
the characteristic dimorphism.
Just what the exact relationship between sex-differentiation and the
_X_-element is has never been clearly established. It is possible that
this element is an actual sex-determinant, in the ordinary cases one
_X_-element determining the male condition and two _X_-elements producing
the female condition. On the other hand it might be argued that it is not
the determining factor but the expression of other cell activities which
do determine sex; that is, a sex accompaniment. Or again, it may be one of
several essential factors which must cooperate to determine sex.
SEX-LINKED CHARACTERS
The discovery of the remarkable behavior of certain characters in heredity
which can only be plausibly explained by supposing that they are linked
with a sex-determining factor still further strengthens our belief in the
existence of such a definite factor. Such characters are commonly termed
sex-linked characters.
=Sex-Linked Characters in Man.--=Since there are a number of them in man
we may choose one of these, such as color-blindness, for illustration. The
common form of color-blindness known as Daltonism in which the subject
can not distinguish reds from greens, a condition which seems to be due to
the absence of something which is present in individuals of normal color
vision, is far commoner in men than in women. Its type of inheritance,
sometimes termed "crisscross" heredity, has been likened to the knight
moves in a game of chess. The condition is transmitted from a color-blind
man through his daughter to half of her sons. Or, to go more into detail,
a color-blind father and normal mother have only normal children whether
sons or daughters. The sons continue to have normal children but the
daughters, although of normal vision themselves, transmit color-blindness
to one-half of their own sons. If such a woman marries a color-blind man,
as might easily happen in a marriage between cousins, then as a rule
one-half her daughters as well as one-half her sons will be color-blind.
[Illustration: FIG. 14
Diagram illustrating the inheritance of a sex-limited character such as
color-blindness in man on the assumption that the factor in question is
located in the sex-chromosome (from Loeb after Wilson). The normal
sex-chromosome is indicated by a black =X=, the one lacking the factor for
color perception, by a light X. It is assumed that a normal female is
mated with a color-blind male.]
In such cases what appears to be a mysterious procedure becomes very
simple if we assume that the defective character is associated with the
sex-determining factor, or to make it concrete let us say with the
_X_-element. The chart shown in Fig. 14, p. 62, indicates what the
germinal condition would be under the circumstances. The column to the
right represents the maternal, the one to the left the paternal line.
Since two _X_ means female and one _X_ male, and inasmuch as we have
assumed that the physical basis of the defect to which color-blindness is
due is conveyed by the _X_-element, we may represent the defective single
_X_ of the male in outline only (see first row). It is obvious that after
the reduction divisions (second row) the mature sex-cells of the female
will each contain a single normal _X_, the corresponding sex cells of the
male will contain either no _X_ or a defective _X_. Since if any member of
the class of spermatozoa containing no _X_, fertilizes an egg the
resulting zygote (row three) will have but one _X_ and that a normal one,
the individual which develops from the zygote will be normal as regards
color vision and moreover will be male because the condition one _X_
always means maleness. On the other hand, if any member of the class of
spermatozoa containing the defective _X_ fertilizes an egg two
_X_-elements are brought together and this of itself means femaleness. In
this case one of the _X_-elements is defective but the single normal _X_
is sufficient in itself to produce normal color vision. But when it comes
to the maturation of the sex-cells of this female, the pair of
_X_-elements are separated in the usual way with the result that half of
the mature ova contain a normal _X_ and half a defective _X_ (row four).
Since in a normal male, however, the mature reproductive cells will
contain either a normal _X_ or no _X_ (fourth row), any one of four
different kinds of matings may result. A sex-cell carrying normal _X_ of
the male may combine with an ovum containing normal X producing a normal
female (row five). Or such a cell may combine with an ovum carrying the
defective _X_, also producing a female but one who although of normal
color vision herself, like her mother, is a carrier of the defect. On the
other hand, any one of the spermatozoa without an _X_ may combine with an
ovum containing the normal _X_, in which case a normal male is produced
and, moreover, one who, like his mother's brothers, is incapable of
transmitting the defect. However, the sperm-cell devoid of an _X_ is just
as likely to fertilize an ovum carrying the defective _X_, in which event
the resulting individual, a male, must be color-blind because he contains
the defective _X_ alone. In other words, the chances are that one-half the
sons of a woman whose father was color-blind will be color-blind, the
other half perfectly normal; and that all of the daughters will be of
normal color vision although one-half of them will probably transmit the
defect to one-half of their sons. From a glance at the diagram it is
readily seen also that a color-blind female could result from the union of
a color-blind man (see first row) and the daughter of a color-blind man
(see third row). For half of the gametes of such a female would bear the
defect as would also that half of the gametes of the male which carry _X_,
hence the expectation would be that half of the daughters of such a union
would be color-blind and half would be carriers of color-blindness; and
that half of the sons would be color-blind and half normal. All the sons
of a color-blind woman would be color-blind because she has only defective
_X_-elements to pass on.
The inheritance of various other conditions in man follows more or less
accurately the same course as color-blindness. Among these may be
mentioned: _hemophilia_, a serious condition in which the blood will not
clot properly, thus rendering the affected individual constantly liable to
severe or fatal hemorrhage; near-sightedness (_myopia_) in some cases; a
degenerative disease of the spinal cord known as _multiple sclerosis_;
progressive atrophy of the optic nerve (_neuritis optica_); Gower's
_muscular atrophy_; some forms of _night-blindness_; in some cases
_ichthyosis_, a peculiar scaly condition of the skin. In one of my own
tabulations of a case of inheritance of "webbed" digits or _syndactyly_, a
condition in which two or more fingers or toes are more or less united, a
sex-linked inheritance is clearly indicated (Fig. 15), although from the
pedigrees recorded by other investigators this condition usually appears
in some of both the sons and daughters of an affected individual.
[Illustration: FIG. 15
Chart showing the inheritance of a case of syndactyly after the manner of
a sex-linked character. The affected individuals are represented in black;
squares indicate males, circles females. The condition is seen to be
inherited by males through unaffected females.]
=The Occurrence of Sex-Linkage in Lower Forms Renders Experiments
Possible.--=The course followed by such characters in man can be inferred
only from the pedigrees we can obtain from family histories. Fortunately,
however, such sex-linkage also occurs in lower animals and we are able
therefore to verify and extend our observations by direct experiments in
breeding. Several sex-linked characters have been found to exist in a
small fruit-fly known as _Drosophila_. Extensive breeding experiments with
this fly by Professor T. H. Morgan and his pupils have borne out
remarkably the interpretation that the characters in question are really
linked with a sex-determining factor.
CHAPTER III
MENDELISM
=New Discoveries in the Field of Heredity.--=Writing in 1899, one of
America's well-known zoologists asserts that, "It is easier to weigh an
invisible planet than to measure the force of heredity in a single grain
of corn." And yet only two or three years later we find another prominent
naturalist saying regarding heredity that, "The experiments which led to
this advance in knowledge are worthy to rank with those that laid the
foundation of the atomic laws of chemistry." Again, "The breeding pen is
to us what the test-tube is to the chemist--an instrument whereby we
examine the nature of our organisms and determine empirically their
genetic properties." Here is a decided contrast of statement and yet both
were justifiable at the time of utterance. For even at the writing of the
first statement the investigations were in progress which, together with
the rediscovery of certain older work, were to transfer our knowledge of
heredity from the realm of speculation to that of experiment and disclose
certain definite principles of genetic transmission.
Through a knowledge of these principles in fact, the shifting of certain
characters is reducible to a series of definitely predictable proportions
and the skilled breeder may proceed to the building up of new and
permanent combinations of desirable characters according to mathematical
ratios and, what is of equal importance, he can secure the elimination of
undesirable qualities. While there are many limitations in the application
of these principles and while new facts and modifications are constantly
being discovered concerning them, nevertheless they represent the first
approximations to definite laws of hereditary transmission that we have
ever been able to make, and the practical fact confronts us that whatever
our theoretical interpretations may be, the principles are so definite
that through their application important improvements of crops and
domesticated animals have already actually been secured and one may
confidently expect still others to follow.
=Mendel.--=The principles involved are called the Mendelian principles
after their discoverer, Gregor Johann Mendel, abbot of a monastery at
Bruenn, Austria. After eight years of patient experimenting in his cloister
garden with plants, chiefly edible peas, he published his results and
conclusions in 1866, in the _Proceedings of the Natural History Society of
Bruenn_. While known to a few botanists of that day, the full importance of
the contribution was not recognized, and in the excitement of the
post-Darwinian controversy, the facts were lost sight of and ultimately
forgotten.
=Rediscovery of Mendelian Principles.--=In 1900 three men, Correns, De
Vries and Tschermak, working independently--in different countries, in
fact--rediscovered the principles and called attention anew to the
long-forgotten work of Mendel which they had come upon in looking over the
older literature on plant breeding. These investigators added other
examples from their own experiments. Since their rediscovery the
principles have been confirmed in essential features and extended by
numerous experimentalists with regard to a wide range of hereditary
characters in both animals and plants.
=Independence of Inheritable Characters.--=It has been found that many
truly heritable characteristics or traits of an individual, whether plant
or animal, are comparatively independent of one another and may be
inherited independently. Where there are contrasted characters in father
and mother, such as white plumage and black plumage in fowls, smooth coat
and wrinkled coat in seed, horns and hornlessness in cattle, long fur and
short fur in rabbits, beard and beardlessness in wheat, albino condition
and normal condition, etc., there is obviously a bringing together of the
determiners of the two traits in the resulting offspring. In the third
generation, however, in the progeny of these offspring, the two distinct
characters may be set apart again, thus showing that in the second
generation while perhaps one only was visible, the factors which determine
both were nevertheless present, and moreover, they were present in a
separable condition.
=Illustration of Mendelism in the Andalusian Fowl.--=Let us take as a
simple example the case of the Andalusian fowl. Although it is not a case
established by Mendel it illustrates certain of the essential conditions
underlying Mendelism in a more obvious way than the cases worked out by
Mendel himself. The so-called blue Andalusian fowl results from a cross of
a color variety of the fowl which is black with one which is white with
black-splashed feathers. The result is the same irrespective of which
parent is black. When bred with their like, whether from the same parents
or different parents, these blue fowls produce three kinds of progeny,
approximately one-fourth of which are black like the one grandparent,
one-fourth white like the other grandparent, and the remaining half, blue
like the parents (Fig. 16). Moreover, the black fowls obtained in this way
will, when interbred, produce only black offspring and the same is true of
the white fowls. To all appearances as far as color is concerned they are
of as pure type as the original grandparents. With the blue fowls,
however, the case is different, for when bred together they will produce
the same three kinds of progeny that their parents produced and in the
same proportions. Again the white and the black are true to type but the
blue will always yield the three classes of offspring and this through
generation after generation.
[Illustration: FIG. 16
Diagram showing the scheme of inheritance in the blue Andalusian fowl.]
These facts may be illustrated graphically as follows where the word
"black" indicates the original black parent, "white" the original white
(black splashed) parent and "blue" the hybrid offspring.
Parental
Generation (P) Black x White Black x White
| |
First Filial | |
Generation (F_{1}) Blue x Blue
|
+------------------------------------------+
| | |
Second Filial Black Blue White
Generation (F_{2}) (25%) (50%) (25%)
| | |
| +-------------------------+ |
| | | | |
Third Filial Black Black Blue White White
Generation (F_{3}) | (25%) (50%) (25%) |
| | | | |
| | +-------------+ | |
| | | | | | |
Fourth Filial Black Black Black Blue White White White
Generation (F_{4}) (25%) (50%) (25%)
=The Cause of the Mendelian Ratio.--=Concerning the cause of this peculiar
ratio of inheritance in crossed forms Mendel suggested a simple
explanation. Animals or plants that can be cross-bred, obviously must be
forms that produce a new individual from the union of two germ-cells, one
of which is provided by each parent. Mendel's idea was that there must be
some process of segregation going on in the developing germ-cells of each
hybrid whereby the factors for the two qualities are set apart in
different cells with the result that half of the germ-cells of a given
individual will contain the determiner of one character and half, the
determiner of the other. That is, a given germ-cell carries a factor for
one or the other of the two alternate characters but not the factors for
both. In a plant, for example, in the male line, half of the pollen grains
would bear germ-cells carrying the determiner of one character and half,
that of the other. Similarly, in the female line, half of the ovules would
contain the determiner of the one character and half, that of the other.
Likewise in animals as regards such pairs of characters there would be two
classes of germ-cells in the male and two in the female. In the case of
the blue Andalusian fowls under discussion this would mean that half of
the mature germ-cells of the male carry the black-producing factor, and
half carry the white-producing factor, and the same is true of the
germ-cells of the female. Thus when two such crossed forms are mated,
there are, by the laws of chance, four possible combinations, namely: (1)
white-determining sperm-cells and white-determining ovum; (2)
white-determining sperm-cells and black-determining ovum; (3)
black-determining sperm-cells and white-determining ovum; and (4)
black-determining sperm-cells and black-determining ovum. Manifestly, the
first combination can only give white offspring; the second, white and
black, gives blue (by such a cross the original blues were established);
likewise, the third, black and white, gives blue; and the fourth
combination can only give black offspring. This matter may be graphically
represented by the following formulae in which B indicates the determiner
of Black in the germ-cell and W the determiner of White: [male] signifies
male; [female] female.
IN THE ORIGINAL PARENTS
W x B = WB = Blue
IN THE HYBRIDS
[male] [female]
germ- germ-
cells cells [male] [female]
W----W W x W = WW = White
\/ W x B = WB = Blue
/\ or B x W = BW = Blue
B----B B x B = BB = Black
Thus of the four possible combinations one only can produce white fowls,
two (WB or BW) can produce blue fowls, and one black fowls. That is, the
ratio is 1:2:1 or the 25, 50 and 25 per cent., respectively, of our
diagram. The black fowls or the white fowls will breed true in subsequent
generations when mated with those of their own color because the
determiner of the alternative character has been permanently eliminated
from their germ-plasm; but the blue fowls will always yield three types of
offspring because they still possess the two classes of germ-cells.
=Verification of the Hypothesis.--=The hypothesis that germ-cells of
crossed forms are of two classes with respect to a given pair of Mendelian
characters is further substantiated by the following facts. If in the case
of the fowls under discussion one of the blue fowls is mated with an
individual of the white variety, half of the progeny will be blue and
half white. For the hybrid has two kinds of germ-cells, black producing,
which we have designated by the letter B, and white producing (or W) in
equal number while the white parent has only one kind, white producing. It
is obvious that if half the germ-cells of the hybrid form are of the type
B then half the progeny will be of the BW type, which is blue, and the
other half will be of the WW type, which is white. In the same way if we
mate a hybrid and a black fowl, half of the progeny will be black and half
will be blue, that is, there could only be WB and BB types.
The fact must not be lost sight of that since the pairings are wholly
determined by the laws of chance the proportions are likely to be only
approximate. It is obvious that the greater the number of individuals, the
nearer the results will approach the expected ratio.
DOMINANT AND RECESSIVE
=One Character May Mask the Other.--=In a large number of cases, however,
the actual condition of affairs is not so evident as in the Andalusian
fowl, for instead of being intermediate or different in appearance, the
generation produced by crossing resembles one parent to the exclusion of
the other. Such an overshadowing is spoken of as _dominance_, and the two
characters are termed _dominant_ and _recessive_. Thus when brown
ring-doves and white ring-doves are mated the progeny are all brown, or if
wild gray mice are mated to white mice the progeny are all gray. So black
is dominant to white in rose-comb bantams; brown eyes to blue eyes in man;
beardlessness to beard in wheat, and likewise rough chaff to smooth, and
thick stem to thin; tallness to dwarfness in various plants; normal
condition to the peculiar waltzing condition in the Japanese waltzing
mouse. Numerous other cases might be cited but these are sufficient to
illustrate the condition.
=Segregation in the Next Generation.--=But now the question arises, what
do such crosses as show dominance transmit to the next generation?
Experiments show regarding any given pair of these alternate characters
that they are set apart again in the succeeding generation, returning in a
definite percentage to the respective grandparental types.
[Illustration: FIG. 17
Diagram showing the scheme of inheritance in guinea-pigs when black and
albino forms are crossed.]
=Dominance Illustrated in Guinea-Pigs.--=In guinea-pigs for example (Fig.
17), when an individual (either male or female) of a black variety, is
crossed with one of a white variety, the F_{1} generation are all black
like the black parent. When these are interbred or bred with other blacks
which have had one black and one white parent, only two visible types of
progeny appear, viz., black and white, and these approximately in the
ratio of three to one.
Analysis by further breeding shows, however, that there are in reality
three types, but since dominance is complete the pure extracted dominant
and the mixed dominant-and-recessive type are indistinguishable to our
eye. That is, while the blacks are three times as numerous as the whites,
two out of every three of these blacks are really hybrid and correspond to
the blue fowls of our former example.
The condition is readily comprehended when expressed diagrammatically
thus:
Generation P Black x White Black x White
| |
Generation F_{1} Black (White) x Black (White)
|
+-------------------------------------+
| | |
Generation F_{2} 1 Black 2 Black (White) 1 White
In other words, the germ-cells of the one original parent (Gen. P) would
contain only determiners for black and that of the other parent would
contain only determiners for white. The condition of the individuals
produced by the cross would be represented by the formula B(W). But these
determiners segregate in the germ-cells of the crossed form, whether it be
male or female, into B and W. Hence half the spermatozoa of the male
hybrid (generation F_{1}) would carry the B determiners and half the W
determiners. The same is true of the mature ova of the female hybrid.
Consequently, in mating there are always four equally possible
combinations, viz., BB, B(W), (W)B, and WW. Since B is always dominant
three out of the four matings would yield black individuals, or in other
words the ratio would be 3:1.
The pure blacks when mated together will breed true in subsequent
generations, likewise the whites, but the blacks carrying white as a
recessive will yield when interbred the same ratio of whites and black as
did their hybrid parents (Fig. 17, p. 75).
=Terminology.--=As work in the study of Mendelian inheritance has
progressed and expanded the need of a more precise terminology has become
evident and such is gradually being established. Thus Professor Bateson
has coined the term "allelomorph" (Gk. _one another_, and _form_) to
express more exactly what we have thus far been calling a pair of
alternate or opposite characters. In the blue Andalusian fowls discussed,
the white condition in the one parent is the allelomorph of the black
condition in the other. The term generally means one of the pair of
Mendelian characters themselves as expressed in the individual plants or
animals but when the germinal basis of such phenomena is under discussion,
it is sometimes used to refer to the determiners of such characters. And
by determiner is meant simply the condition which is necessary in the germ
to bring about the occurrence of a definite character. For example, when
we are studying a cross between a red flower and a white flower with
reference to the color factors, the difference between the two plants may
lie in the fact that one produces a red coloring matter and the other
does not. That is, the determiner for red is absent from the white
variety. What the exact relation of color production is to the parts of
the germ-cell we do not know. It could be the function of a single
definite body or the resultant of several cooperating bodies. The latter
is far more likely to be the case. We may suppose that a group of
cooperating substances function to produce red in the red flower but that
in the white flowers one of these bodies is absent or fails to perform its
red-producing function.
It is customary where practicable to refer to the determiner of a
character by the initial letter of the name of the character. The letter
when written as a capital indicates the determiner but when written as a
small letter the absence of the determiner. Thus R may be taken to
represent the determiner for red coloring matter and r its absence. It is
convenient also to have a brief symbol to denote a given generation and
for this purpose Bateson has introduced the symbol F_{1} for the hybrid
progeny of the first cross, the initial letter of the word "filial." F_{2}
would indicate the next generation, F_{3} the third and so on. Likewise P
denotes the original parent generation.
=The Theory of Presence and Absence.--=Many, if not all, allelomorphs
consist of the presence and absence respectively of a given determiner. In
such cases the character represented by the presence of the determiner is
dominant over the character represented by the absence of a determiner.
Thus in the crosses from the wild gray mice and albino mice the progeny
are all gray mice since one parent had the determiner or group of
determiners for grayness and the hybrid offspring must also possess it.
Likewise the presence of black in black guinea-pigs is dominant to its
absence in albino guinea-pigs and the resulting progeny are all black.
However, it has already been mentioned that beardlessness in wheat is
dominant to beard and that the absence of horns in cattle is dominant to
their presence, that is, the progeny of hornless by horned cattle are
without horns except for occasional traces of imperfect horns. Facts like
these would seem at first sight to contradict the assertion just made that
presence is dominant to absence, but it is fairly well established that in
such cases one is not dealing with true absences but with suppressions.
The polled breeds of cattle, for example, are hornless not because of the
absence of determiners for horns but because of the presence of an
additional inhibiting factor which prevents these determiners from
functioning. The horned breeds are without this inhibitor. When horned and
hornless individuals are crossed the presence of the inhibitor from one
line of ancestry is sufficient to suppress the development of horns in the
progeny. A similar explanation would, of course, apply to beardlessness in
wheat.
In writing double-lettered formulae to denote the determiners of characters
in hybrids the condition is represented merely by the capital and small
letter. Thus Rr indicates that red is dominant to its absence.
=Additional Terminology.--=In pure breeds where the determiners are alike
as BB in black or bb in albino guinea-pigs, the individual is said to be a
_homozygote_ (like things united) with reference to that character, while
in those in which the determiners are unlike, as Bb, the individual is
termed a _heterozygote_ (unlike things united) with reference to the
character. Or to use the adjective forms, a pure black guinea-pig is
homozygous for black pigment, an albino guinea-pig is homozygous for
absence of pigment, while a cross between the two is heterozygous for
pigment. Also, where the determiner of a given character is present in
double quantity, that is, from both lines of ancestry, the individual is
said to be _duplex_, where represented in only the single form as in
heterozygous individuals, _simplex_, and where the determiner is absent
entirely, _nulliplex_, with reference to the character in question. Thus
black guinea-pigs of formula BB are duplex with regard to the determiner
for black color, individuals of formula Bb are simplex with reference to
this determiner, and those of formula bb are nulliplex.
A heterozygote in which dominance prevails can be identified with
certainty by breeding to a known recessive and noting the kind of
offspring produced. If the individual was really a heterozygote,
approximately fifty per cent. of the offspring should be of the recessive
type.
=Dominance Not Always Complete.--=As a matter of fact close inspection
shows that in numerous instances dominance is not absolute since traces of
the recessive character may be detectable. For example, in the cross
between smooth and bearded wheat while smoothness is regarded as the
dominant character and beardlessness as the recessive, nevertheless in the
hybrid offspring a slight tendency toward bearding is not infrequently
seen. Or again when horned breeds of cattle are crossed with hornless
ones, a small proportion of such progeny will show traces of imperfect
horns.
In some cases instead of either character dominating the other a form
intermediate between the two parents may result, as we have seen already
in the case of the Andalusian fowl. Thus, certain white-flowered plants
and certain red-flowered plants when crossed produce pink hybrids, and
longheaded and shortheaded wheats when crossed give offspring with heads
of intermediate length. Or again, crosses between white and red cattle may
yield red roans, and between black and white cattle, blue roans.
Thus, while for such pairs of alternative characters as have been studied,
dominance to some considerable degrees at least, seems to be the rule,
still we have gradations down to the intermediate condition, and in some
instances the hybrid with respect to a given character may be unlike
either parent. The things of chief importance in the Mendelian discovery
are the independent, unitary nature of the characters and their
segregation in the offspring of cross-bred forms.
=Modifications of Dominance.--=It should be noted also that there is such
a condition as _delayed dominance_. Davenport found, for example, that
chicks produced by crossing pure white with pure black Leghorn fowls are
speckled black and white, but later in the adult form white becomes
dominant. Likewise conditions of delayed dominance are known in man in
eye-color and notably in color of hair. Some few cases have been recorded
where a character is dominant at one time, recessive at another.
According to Davenport extra toe in fowls may behave in this way.
=Mendel's Own Work.--=Mendel[2] himself worked out his principles on seven
pairs of characters which he found in common culinary peas. Placing the
dominant characters first, these may be enumerated as follows: (1) Tall by
dwarf; (2) green pod (unripe) by yellow; (3) pod inflated by pod
constricted between the individual peas; (4) flowers arranged along the
axis of the plant by flowers bunched together at the top; (5) seed skin
colored by seed skin white; (6) cotyledons yellow by cotyledons green; (7)
seed rounded by seed wrinkled.
He found that each pair of characters followed the same law as any other
pair when more than one pair of the characters occurred in the same
plants, but that each pair behaved independently of the other. The meaning
of this is that we may get various combinations of characters not
associated in the original pure stocks, the number of such combinations
depending on the number of pairs of allelomorphs there are.
DIHYBRIDS
=Getting New Combinations of Characters.--=Since this principle is well
illustrated in peas, let us take two pairs of their characters, viz.,
greenness and yellowness (of the cotyledons) and roundness and angularity
to see exactly what happens when two pairs of allelomorphs are involved.
When a specific kind of yellow pea is crossed with a particular kind of
green pea the offspring are always yellow (Fig. 18, opposite p. 84). When
these hybrids (generation F_{1}) are self-fertilized there is the usual
Mendelian segregation; one-fourth the resulting offspring will be green,
one-fourth pure yellow, and one-half, although yellow in appearance, will
be of the mixed type. The exact numbers found by Mendel were 6,022 yellow
seeds to 2,001 green seeds. Now of the original peas (generation P) the
yellow ones are round and the green ones angular (really wrinkled).
Choosing this roundness and angularity respectively as a pair of
characters they are found to follow the same law that the colors follow
(Mendel obtained in the F_{2} generation 5,474 round and 1,850 wrinkled
seed), but independently of the latter. For while in the progeny of the
hybrids (Gen. F_{1}), twenty-five per cent. will be round and of pure type
as regards roundness, twenty-five per cent. angular, and fifty per cent.
round but containing hidden factors of angularity (i. e., roundness is
dominant), the roundness and the yellowness, or the angularity and the
greenness will not always go together as they did in the original
grandparental strains, but there will be in addition some new types of
round green peas and some of angular yellow ones. That is, the factors of
color and of shape have been inherited independently of one another, so
that instead of the two original kinds of peas, four have been produced,
viz., (1) round-yellow (one of the original types); (2) round-green (new
type); (3) angular-yellow (new type); and (4) angular-green (one of the
original types). Furthermore, these will be found to stand in the ratio of
9:3:3:1 respectively.
=Segregations of the Determiners.--=How these combinations come about in
this definite proportion is easily understood if the matter is expressed
in terms of determiners and the possible matings tabulated (Fig. 18). If
we represent the yellow determiner by Y and the green determiner by y, and
likewise the determiners of roundness and angularity by R and r
respectively, then the formulae for the determiners of these two pairs of
characters in the body cells (that is, in the unreduced condition) of the
pure forms and of the F_{1} generation hybrids respectively are as
follows:
In pure round yellow peas RR YY
In pure angular green peas rr yy
In the hybrid Rr Yy
But now in the segregation of these determiners in the germ-cells of the
hybrids (generation F_{1}) the pair of determiners Rr and the pair Yy
operate entirely independently of one another. Their only compulsion is
that each pair be separated into the single determiners, R and r in the
one case and Y and y in the other. So in the separating division which
brings about this divorcement R separates from r irrespective of whether
it is accompanying Y or y into the resulting daughter cell. Thus in some
cases R and Y would pass into one germ-cell, in others R and y, in others
r and Y, and in still others r and y, depending entirely upon the
chance relations of the respective pairs to the plane of division. That
is, the segregation is equally likely to be RY/ry giving gametes RY and
ry, or Ry/rY giving gametes Ry and rY.
[Illustration:
\[Female]
[Male]\ RY Ry rY ry
+-------+--------+--------+--------+
RY | RRYY | RRYy | RRYY | RrYy |
+-------+--------+--------+--------+
Ry | RRYy | RRyy | RrYy | Rryy |
+-------+--------+--------+--------+
rY | RrYY | RrYy | rrYY | rrYy |
+-------+--------+--------+--------+
ry | RrYy | Rryy | rrYy | rryy |
+-------+--------+--------+--------+
(1) 1 RRYY (4) 2 RrYY (7) 1 rrYY
(2) 2 RRYy (5) 4 RrYy (8) 2 rrYy
(3) 1 RRyy (6) 2 Rryy (9) 1 rrYy
9:3:3:1
FIG. 18
Diagram showing the possible combinations arising in the second filial
generation (F_{2}) following a cross between yellow, round (YYRR) and
green, angular or wrinkled (yyrr) peas. Y, presence of factor for yellow;
y, absence of such a factor; R, presence of factor for smoothness or
roundness; r, absence of such a factor; [male] male; [female] female.]
=Four Kinds of Gametes in Each Sex Means Sixteen Possible
Combinations.--=There are, therefore, with reference to the two pairs of
characters under consideration, four kinds of gametes (or mature
germ-cells) produced in equal numbers in each hybrid, viz., RY, Ry, rY,
and ry. That is, in the first type roundness and yellowness are
associated, in the second roundness and greenness, in the third angularity
(lack of roundness) and yellowness, and in the fourth angularity and
greenness.
But since both males and females have these four kinds of gametes, when
they are mated there will be sixteen possible combinations. These may be
tabulated as in Fig. 18, opposite p. 84.
=The 9:3:3:1 Ratio.--=While there are sixteen possible and equally
probable combinations, these will give only nine distinct kinds because
some of the matings are alike. The numbers of the various kinds of matings
are as follows:
(1) 1 RRYY (4) 2 RrYY (7) 1 rrYY
(2) 2 RRYy (5) 4 RrYy (8) 2 rrYy
(3) 1 RRyy (6) 2 Rryy (9) 1 rrYy
Since roundness (R) and yellowness (Y) are dominant to angularity (r) and
greenness (y) in all combinations containing R or Y, the alternative
determiners r or y would be obscured, with the result that individuals
having certain of the combinations would look alike to our eye. For
example, the individuals represented by numbers 1, 2, 4 and 5, since they
contain dominant R and Y, would all appear round and yellow, although in
reality No. 1 would be the only one of pure type (both elements
homozygous) and hence the only one that would breed true in subsequent
generations. The two individuals represented in No. 2 would breed true as
regards shape (RR) but not color (Yy). Just the reverse is true of No. 4
since shape is heterozygous (Rr) and color homozygous (YY). The four
individuals represented in No. 5 are heterozygous with regard to both
elements. Thus nine individuals (1 plus 2 plus 2 plus 4 = 9) represented
in Nos. 1, 2, 4 and 5 would be round and yellow, three individuals (Nos. 3
and 6) would be round and green, three (Nos. 7 and 8) would be angular and
yellow, and only one (No. 9) would be angular and green. That is to say,
the four classes discernible to the eye in generation F_{2} would be
present in the ratio of 9:3:3:1.
=Phenotype and Genotype.--=Forms such as those represented in Nos. 1, 2, 4
and 5 which to the eye appear to be alike, regardless of their germinal
constitution, are said to be of the same _phenotype_. Those of the same
hereditary constitution, as the two individuals represented in No. 8, or
the four individuals in No. 5, are said to be of the same _genotype_, that
is, they are of identical gametic constitution.
As we have seen, it is from the genotypical not the phenotypical
constitution that an offspring is derived and what a given form will bring
forth depends then on its genotype.
=Crosses With More Than Two Pairs of Characters.--=In crosses in which
more than two pairs of contrasted characters are involved the underlying
principles are in no way different, only with each pair of additional
characters there is, of course, a greater number of possible combinations.
Thus with three pairs of characters there will be eight different classes
of gametes in each sex and consequently sixty-four possible combinations
in mating, giving eight different phenotypes in the proportion of
27:9:9:9:3:3:3:1. The largest class manifests the three dominant
characters; the smallest class, the three recessives; the three classes in
the proportion of 9 each exhibit two dominant and one recessive
characters; and those in the proportion of 3 each display two recessive
and one dominant characters.
THE QUESTION OF BLENDED INHERITANCE
We come now to certain types of inheritance in which there seems to be a
true fusion or blend of the contributions from the two parents, the
intermediate condition apparently persisting in subsequent generations
without segregation. Numerous cases of blended inheritance have been cited
in earlier literature of heredity, but as our knowledge of genetics has
progressed many experimental breeders have come to believe that the
blends in such cases are apparent rather than real and that the phenomena
can be best explained on a non-blending unit-character basis, just as we
would explain ordinary Mendelian phenomena.
=Nilsson-Ehle's Discoveries.--=To get their point of view we may review
certain experiments on wheat made by Nilsson-Ehle, together with their
Mendelian interpretation. Nilsson-Ehle found that a certain brown-chaffed
wheat when crossed with a white-chaffed strain yielded a brown-chaffed
hybrid, apparently in accordance with the simple principle of Mendelian
dominance. But these heterozygous brown-chaffed individuals did not in
turn give the expected ratio of 3:1 in the F_{2} generation but a ratio of
15 brown to 1 white, and furthermore the browns were not all of the same
degree of brownness. To be exact, from fifteen different crosses of the
strains he obtained 1,410 brown-chaffed and 94 white-chaffed plants.
This apparent anomaly in segregation was easily explained, however, when
it was finally figured out that there were really two independent
determiners for brown color, either of which alone could produce a brown
individual, but when combined produced individuals of correspondingly
deeper shades of brown. In such a case then Nilsson-Ehle discovered that
he was dealing merely with a Mendelian dihybrid where two different
determiners B and B' and their respective absences b and b' are involved.
The original brown wheat had both B and B' and the original white b and
b'. The formula for the F_{1} heterozygote was therefore BbB'b'. The four
possible types of gametes for male and female are BB', Bb', bB', bb', and
the tabulation would be as follows:
+-----------------------------------
| BB' | Bb' | bB' | bb'
-----+--------+--------+--------+--------
BB' | BBB'B' | BBB'b' | BbB'B' | BbB'b'
-----+--------+--------+--------+--------
Bb' | BBB'b' | Bb'Bb' | BbB'b' | Bbb'b'
-----+--------+--------+--------+--------
bB' | BbB'B' | BbB'b' | bbB'B' | bbB'b'
-----+--------+--------+--------+--------
bb' | BbB'b' | Bbb'b' | bbB'b' | bbb'b'
-----------------------------------------
It will be observed that there are more brown determiners in some
combinations than others. For instance one of the sixteen contains four
such determiners, viz., B, B', B, B', four contain three determiners, six
contain two, four contain only one, and one contains none. Thus all but
one of the sixteen contain at least one determiner and will therefore be
brown in color but the depth of color will depend on the number of brown
determiners in a given individual. This is more graphically represented in
Fig. 19, p. 90. The largest number of similar individuals, six in all,
contain two determiners each and represent an intermediate "blend" between
the original brown-chaffed and white-chaffed strains. The deeper and the
lighter browns due to more or fewer determinants in an individual would if
one did not know the units in this case look like the fluctuations around
this average which we might expect in a blend.
[Illustration: FIG. 19
Diagram illustrating the proportionate distribution of determiners where
either of two different determiners produces the same character, the
degree of expression of the character depending on the number of the
determiners present. The numerals indicate the number of brown determiners
present in an individual.]
Nilsson-Ehle found another significant case in wheat where one particular
red-grained strain of Swedish wheat when crossed with white-grained
strains produced red-grained offspring, but when these were interbred the
F_{2} generation gave approximately sixty-three red to one white-grained
individual. Here it was found that in the original red wheat there are
three separate determiners which act independently of one another in
heredity, any one of which would make red color; and that they together
with their absences simply follow the Mendelian laws for a trihybrid.
=Such Cases Easily Mistaken for True Blends.--=If we should tabulate the
possible combinations as we did the dihybrid we should see that we would
get individuals having varying numbers of red determiners. Only one of the
sixty-four possible combinations would be without a factor for red. Of the
sixty-four, one would have six determiners for red, six would have five,
fifteen would have four, twenty would have three, fifteen would have two,
six would have one, and one would have none. Since here every additional
red factor means deeper redness in the individual there would be varying
degrees of redness in the F_{2} generation with those having three
determiners, the largest group, standing apparently intermediate. Not
knowing the factors involved we might easily mistake such a case for a
true blend with fluctuations about an average intermediate form.
Nilsson-Ehle finally proved his interpretation by rearing an F_{3}
generation from isolated and self-fertilized plants of this F_{2}
generation.
This same principle of cumulative determiners has also been established in
America by East with field corn.
As the number of duplicate determiners increases it can be readily seen
that the number of apparent blends of different degrees of intermediacy
between the two extremes would rapidly increase.
=Skin-Color in Man.--=In man, the skin-color of the hybrids between
negroes and whites is often cited as a case of blended inheritance in
contradistinction to Mendelian inheritance. The skin-color of the mulatto
of the F_{1} generation is intermediate between that of the white and
black parent. This same degree of intermediacy is commonly supposed to
persist in subsequent generations, but as a matter of fact, careful
investigation has shown that while mulattoes rarely produce pure white or
pure black children, there is considerably greater range in the shades of
color in the F_{2} generation and subsequent generations than in the F_{1}
generation. This is exactly what one would expect of a Mendelian character
in which several cooperating factors were involved. Indeed, Davenport who
has made extensive studies[3] on the inheritance of skin-color in man has
come to the conclusion that the case is really one of Mendelian
inheritance in which several factors for skin-color are concerned. Even
the skin of a white man is pigmented in some degree under normal
conditions. Davenport has shown in the skin of both whites and blacks that
there is a mixture of black, yellow and red pigments. He concludes that
"there are two double factors (AABB) for black pigmentation in the
full-blooded negro of the west coast of Africa and these are separably
inheritable." Since these factors are lacking in white persons the
intermediate color of an F_{1} mulatto would therefore be heterozygous for
pigmentation, and subsequent generations, following the laws for
segregation where a number of factors are concerned, would show different
degrees of color because of the varying combinations of factors.
=Some Investigators Would Question the Existence of Real Blends.--=Still
other reputed blends such as ear length in rabbits and the like have been
shown to be analyzable into Mendelian behavior if one will but postulate
numerous or multiple factors. Just how far we are justified in so
accounting for blends has not yet been established. Some of our most
careful experimentalists in heredity still believe that real blends exist,
particularly where the character is quantitatively expressed--that is, as
more or less of a given size or amount--while others would maintain that
all alleged blends will probably be found to be resolvable into factors
which follow Mendelian rule. It must be left for future investigations to
demonstrate which school is correct.
THE PLACE OF THE MENDELIAN FACTORS IN THE GERM-CELLS
=Parallel Between the Behavior of Mendelian Factors and Chromosomes.--=The
question arises as to whether there is any evidence from the study of
germ-cells themselves to bear out the Mendelian conception of separation
of contrasted characters in the gametes of the F_{1} generation. In the
discussion of the maturation of germ-cells (Chap. II) it has already been
seen that the chromosomes of the germ-cells are in all probability
arranged in homologous pairs, one member being of maternal and the other
of paternal origin, and that furthermore they are closely associated with
the phenomena of heredity. And since in maturation there is an actual
segregation of the chromosomes into two sets, half going to one cell and
half to its mate, a physical basis adequate to the necessities of the case
is really at hand. It will be recalled that the individuals of a pair
separate in such a way at the reduction division that the paternal member
goes to one cell and the maternal member to the other, although each pair
seemingly acts independently of the others with the result that any mature
germ-cell may contain chromosomes from each of the original parents but
never the two chromosomes which earlier made up a pair. The close parallel
between the behavior of chromosomes and the behavior of Mendelian factors,
although the two sets of phenomena were discovered wholly independently of
each other, is obvious. If we suppose that each chromosome bears the
determiner of a Mendelian character and that chromosomes bearing
allelomorphic characters make up the various pairs which are seen in the
early germ-cells of an individual before reduction occurs, then the
segregation of the individuals of an allelomorphic pair into different
gametes must result in consequence of the passing of the corresponding
chromosomes into separate gametes. Fig. 20, p. 95, from Professor Wilson
represents equally well the segregations of pairs of chromosomes or pairs
of Mendelian characters.
[Illustration: FIG. 20
Diagram showing union of factors from the two separate parents in
fertilization and their segregation in the formation of germ-cells (after
Wilson). With four pairs of factors (_Aa_, _Bb_, _Cc_, _Dd_), sixteen
types of gametes are possible, as shown in the series of small circles at
the right. The same diagram equally well represents the pairings and
segregations of chromosomes.]
=A Single Chromosome not Restricted to Carrying a Single Determiner.--=It
has been objected that there may be more pairs of independently heritable
allelomorphic characters than there are pairs of chromosomes. It is true
that there are more pairs of characters than pairs of chromosomes but
there is no reason for supposing that a given chromosome is restricted to
carrying a single unit-determiner. On the contrary it probably carries
several or many. Some workers have pointed out that certain units might be
interchanged during the pairing of chromosomes before the reduction
division, others that inasmuch as the chromosomes become diffuse and
granulated during the intervals between divisions it is not improbable
that the individual units may become separated from their original system
during such times and that it is a matter of chance into which of the
homologous chromosomes, A or a, they enter with the re-establishment of
the chromosomes. On the other hand, cases are known where two or more
separate characters are permanently associated in inheritance, that is, if
they enter a crossed form together they come out together in the
grandchildren as if they were carried in the same unit-body in the
germ-cell. The only observable unit-bodies that fulfil the necessities of
such cases are the chromosomes. This tendency of characters to exist in
groups which are inherited independently of one another is coming more and
more into evidence as we penetrate farther into the intricacies of
inheritance, and it is exactly what we would expect on the supposition
that each chromosome carries the determiners of a number of characters
instead of a single one.
CHAPTER IV
MENDELISM IN MAN
=The Mendelian Principles Probably Applicable to Many Characters of
Man.--=We are really just beginning to make the proper observations and
collect the necessary data with reference to the application of Mendelian
principles to the traits of man. Yet brief as has been our study we have
disclosed much significant evidence which makes it seem highly probable
that many of his characters, good and bad, of mind and body are as
subservient to these laws as are the traits and features of lower forms.
Davenport and Plate record over sixty human characters or defects which
are seemingly inherited in Mendelian fashion. Although about fifty of
these are pathological or abnormal conditions, this does not mean that
such conditions are more prone to follow Mendelian inheritance but merely
that being relatively conspicuous or isolated they are easier to follow
and tabulate.
=Difficult to get Correct Data.--=While it must be said that in many cases
no simple form of Mendelian tabulation has been unequivocally established,
yet the general behavior of the various inheritable traits in question is
so obviously related to the conventional Mendelian course that there seems
little reason for doubting that they are at bottom the same. Failure to
obtain exact proportions may be attributable in part to the probability
that what we loosely regard as a character should in reality be analyzed
into more elemental components, and above all to the fact that from the
very nature of the conditions under which human records must be obtained,
there is considerable chance of inaccuracy or error in such accounts. How
many human traits follow Mendelian rules remains largely for future
investigators to establish.
We are handicapped at the outset in man by the many difficulties of
getting correct data from the genealogies on which we must depend, or in
fact of getting any genealogy at all, for in this country at least, most
families keep imperfect records of births and deaths and many of the
institutions for the various kinds of defectives have little in their
records that will help us in following out hereditary conditions. Then in
matters of disease we meet with the fact that many former diagnoses were
erroneous. In yet other cases, and this is particularly true among mental
and moral defectives, we are often not sure of the paternity of a given
child. Furthermore, one is likely to be misled by the proportions which
may occur in the very limited number of children of any given couple.
Still other difficulties exist. Among these is the fact, for example, that
in many cases of defect or susceptibility to disease, a given individual
in the stock may have the trait in an expressible and transmissible form,
yet it never comes to expression because that individual has been
fortunate enough to escape the environmental stimulus which would call it
forth. Thus one highly susceptible to tuberculosis might escape infection,
or persons hovering on the verge of insanity might never receive the
precipitating stimulus which would topple them into actual insanity; yet
each would be wrongfully recorded in a genealogy looking to such traits as
perfectly normal. Or again if it be a question of intellectual brilliancy
as shown by accomplishment in the realm of scholarship, or of worldly
affairs, the ones who although possessing them have had no chance to
display unusual talents would be tabulated as average whereas in fact they
should be recorded as of high rank. That this is particularly likely to
happen in the case of women is evident.
=A Generalized Presence-Absence Formula for Man.--=In man as in lower
forms some characters or traits are due presumably to the presence of
determiners or to their absence. Likewise, dominance and recessiveness are
as much in evidence, for in tracing back pedigrees of various traits we
find the same forms of tabulation that obtain for these conditions in
plants and lower animals hold good. For typical cases in man let us use a
generalized presence-absence formula and the arbitrary symbol A for the
presence of the determiner of the character (double in the individual,
single in the germ) and a for its absence. Thus AA represents a condition
in which similar determiners have been derived from both parents and the
individual is _duplex_ as regards the character in question; each mature
germ-cell will have the determiner. Aa represents a condition in which the
individual has received the determiner from only one parent and is
therefore _simplex_ with regard to the character; half of the gametes of
such an individual will have the determiner and half will lack it. Lastly,
aa represents total absence of the determiner. Such an individual is
_nulliplex_. He or she will not have the determiner represented in any of
the gametes, and can not, of course, transmit a trait represented by the
determiner.
It is evident that six kinds of gametic matings are possible among
individuals representing these various formulae. These matings are as
follows:
Possible
couplings
Matings of gametes Product
1. Nulliplex x Nulliplex (aa x aa) == a------a == all nulliplex
\/
/\
a------a
2. Nulliplex x Simplex (aa x Aa) == a------A == 50 per cent.
\/ with character
/\ nulliplex and
a------a 50 per cent.
with it
simplex.
3. Nulliplex x Duplex (aa x AA) == a------A == all with characters
\/ simplex
/\
a------A
4. Simplex x Simplex (Aa x Aa) == A------A == 25 per cent.
\/ with characters
/\ duplex, 50 per
a------a cent. with it
simplex and 25
per cent. with
it nulliplex.
5. Simplex x Duplex (Aa x AA) == A-------A == 50 per cent.
\/ with character
/\ duplex and 50
a-------A per cent. with
it simplex.
6. Duplex x Duplex (AA x AA) == A-------A == all duplex.
\/
/\
A-------A
=Indications of Incomplete Dominance.--=While in cases of strict Mendelian
dominance it is not possible to distinguish directly the simplex from the
duplex condition, as a matter of fact the individual of simplex
constitution sometimes has the character represented in the single
determiner less perfectly developed than in the corresponding character of
duplex origin. In studying defects in man due to the absence of a
determiner, where theoretically presence of the determiner (normality) is
dominant over its absence in individuals of simplex constitution, one
finds it recorded with increasing frequency that such individuals are more
or less "intermediate" or are "tainted" with the defect; thus showing that
the defect though obscured is not wholly in abeyance. Thus individuals
carrying epilepsy or feeble-mindedness which are regarded as recessive
traits, while not showing specific feeble-mindedness or epilepsy, may
nevertheless apparently show a neuropathic taint in the form of migraine,
alcoholism or other lapse from normality. The condition is seemingly more
akin in some cases to that found in the offspring of certain red flowers
crossbred with white flowers, which though red do not show the same
intensity of color as the original red parent. Just as here the single
determiner or single "dose" of redness is insufficient to produce the
intensity of color that appears when the offspring receive two determiners
for red, one from each parent, so in man a single determiner for normality
of a specific character is inadequate in some cases to make the individual
wholly normal. Or possibly some cases are more of the type of those in
which the character in question, for instance the red color of some wheats
and corn, may be produced by any one of two or three determiners, the
intensity of the characters (red color, e. g.) depending on whether one,
two or three determiners are present.
=Why After the First Generation Only Half the Children May Show the
Dominant Character.--=If the trait is a simple dominant one it is clear
that it will appear in each generation and always spring from an affected
individual. By referring back to our tabulation of possible matings on
page 100 where the dominant character is represented by the letter A, this
can be seen at a glance. If the trait is present in the duplex condition
in one parent and absent from the other, then formula 3 applies; all
children will show the trait, but in the simplex form (Aa). If the trait
is present in the simplex form in one parent and absent in the other,
formula 2 applies. Fifty per cent. of the children will have the character
in the simplex form (Aa) which means also an even chance of transmitting
it to their offspring; fifty per cent. will not inherit it and will be
incapable, furthermore, of transmitting it, since they have become
nulliplex (aa). In human genealogies if an individual having an unusual
trait which is inherited as a dominant marries a normal person and half of
the offspring show the trait (and this is common), this means that the
parent manifesting the trait had it represented only in the simplex
condition, otherwise all of the children would have shown it. Even though
the original ancestor who first developed the condition or structure may
have had it in a duplex form, it would after the first mating, if this
were with an individual lacking the trait, be represented only in the
simplex form (see formula 5) and could never become duplex again unless
two individuals both having the character married, and then only in
twenty-five per cent. of the offspring (see formula 3). If the trait is a
defect all the children showing it, even though marrying normal
(nulliplex) individuals, will pass it on again to half their children, but
those who do not show it may ordinarily marry with impunity since its
non-expression in their make-up means, as far as we know at present, that
their germ-plasm has been purged of the defect and that they are therefore
nulliplex with reference to it.
=Eye-Color in Man.--=Of normal characters in man which follow the
Mendelian formula perhaps eye-color is the best established. Brown or
black eye-color is due to a _melanin_ pigment absent from the blue or gray
eye. That is, a brown eye is practically a blue eye plus an additional
layer of pigment on the outer surface of the iris. The different shades of
brown and the black are due to the relative abundance of this pigment.
Gray color and the shades of blue seem to be a modification of an original
dark blue, due to structural differences in the fibrous tissues of the
iris.
In inheritance brown or black is dominant to blue or gray, or in other
words the _presence_ and _absence_ of a pigment P constitutes a pair of
allelomorphs. Hence two brown-eyed parents, if P is duplex in both (or
duplex in one and simplex in the other) can have only brown-eyed children.
Thus,
1. PP x PP = PP, or all duplex brown.
2. PP x Pp = PP and Pp, half duplex brown and half simplex brown.
If each parent has brown eyes but in simplex condition, then one-fourth
of children will have blue or gray eyes; for example,
Mating Gametic Product
couplings
Pp x Pp = P--P = PP, Pp, pP, and pp, or one-fourth
\/ duplex brown, one-half simplex
/\ brown, and one-fourth blue or
p--p gray.
If both parents have blue or gray eyes they can not have children with
black or brown eyes, since the recessive condition in each parent means
total absence of brown pigment in both.
If one pair is duplex brown and the other blue, then all children will
have brown eyes but of simplex type.
If one parent has simplex brown eyes (type Pp) and one blue (pp) then
one-half of the children will have brown eyes of simplex type and one-half
will have blue eyes.
Occasional objections have been raised against the Mendelian
interpretation of inheritance in eye-color, but the cases cited in
evidence against the theory usually narrow down to those in which the
color is so diluted as to render classification uncertain. For example,
hazel eyes are sometimes called gray; they belong however to the melanic
pigmented type although the brown pigment may be much diluted and occur
mainly around the pupil. So-called green eyes are due to yellow pigment on
a blue background. In the rare cases where in the same individual one eye
is brown and the other blue, the individual should probably be rated as
brown-eyed on the supposition that in the one eye the development of brown
pigment has in some way been suppressed.
=Hair-Color.--=The inheritance of hair-color has also been the subject of
considerable study and while the conditions are not so simple as in the
case of eye-color, there is little doubt that it belongs in the Mendelian
category. In human hair, color has as its foundation apparently two
pigments, black and red. Absence of one or both or various combinations or
dilutions of these seemingly account for the prevailing colors in human
hair. In general dark hair is dominant to light, although because of the
delay sometimes in the darkening of the hair in children this fact is
often obscured. Black is dominant to red. People with glossy black hair,
according to Davenport, are probably simplex for black, the glossiness
being due usually to recessive red. The expectation would be for some of
the children of such a pair to have red hair.
In man occasionally a congenital white lock contrasting strikingly with
the remaining normally pigmented hair occurs. It behaves as a simple
dominant in heredity.
=Hair-Shape.--=Again, straight and curly hair seem to be distinct
inheritable characters. Curly is incompletely dominant to straight, the
simplex condition yielding wavy hair.
Not to enter into details of the matings, statistics gathered by Mr. and
Mrs. Davenport show that, two flaxen-haired parents have flaxen-haired
children; two golden-haired parents have only golden-haired children; two
parents with light brown hair have children with hair of that color or
lighter, but never darker; two parents each with dark brown or black hair
may have children with all the varieties of hair-color. Summing together
a series of recessives Davenport points out that two blue-eyed, flaxen or
golden and straight-haired parents will have only children like
themselves.
[Illustration: FIG. 21
Diagram showing descent of brachydactyly through five generations; black
symbols indicate affected individuals; [male], male; [female], female
(after Farabee).]
=Irregularities.--=If a dominant trait or defect depends on more than a
single factor, as is sometimes the case, or if it is modified by sex or
other conditions, as is true of certain characters, some of which, such as
color-blindness, have already been examined, then we shall find some
apparently non-affected individuals having affected offspring. Certain
diseases, for example, are generally transmitted by affected members of
the family to their children in the expected Mendelian ratio for a
dominant, yet an occasional skip of a generation may appear in which an
apparently perfectly normal individual transmits to his children what,
except for the omission in his own case, appears to be an ordinary
dominant character. This occasional lapse in the appearance of a
character when theoretically it should appear is doubtless due in some
instances to the fact that what is really inherited is a _tendency_, and
although this is present in the apparently normal individual, for some
reason the condition itself has not appeared. This might especially be
true in the case of a disease which does not manifest itself until late in
life. In other cases there are undoubtedly complicating accessory
conditions which modify the behavior of the trait somewhat.
OTHER CASES OF DOMINANCE IN MAN
Among other normal characters in man, as far as available evidence goes,
dark skin is dominant to light skin; normally pigmented condition to
albino; and nervous temperament to phlegmatic.
=Digital Malformations.--=An interesting and easily followed defect is a
condition known as _brachydactylism_, in which the digits are shortened
because of the absence or rudimentary condition of one segment. The
fingers, therefore, appear to be only two-jointed like the thumb. Several
families showing this defect have been charted and it appears to behave as
a typical dominant. In looking over such a chart (Fig. 21, p. 106) one is
struck by the fact that only half of the children from most of the matings
show the defect, but when we recall that the affected parent, after the
first generation, probably carried the condition in only the simplex form
and married a normal individual, such a result is just what would be
expected (see formula 2).
_Polydactylism_ (Figs. 22, 23, pp. 109, 110) is a condition in which there
are extra digits on hands or feet. The character, with apparently slight
exceptions in a few records, behaves as a typical dominant. Among other
digital defects which are inherited as a dominant is a condition known as
_syndactylism_ (Fig. 24, p. 111), in which two or more digits are fused
side by side. For an example of syndactyly which seems to be in the class
of sex-linked characters, see Fig. 15, p. 65.
=Eye Defects.--=_Congenital cataract_ is another not uncommon defect in
man which is transmitted as a dominant (Fig. 25, p. 112) with occasional
irregularities. It is a condition of opacity of the lens of the eye which
produces partial or total blindness. In a paper on _Hereditary Blindness
and Its Prevention_, Clarence Loeb (1909) mentions 304 families of which
pedigrees have been published. Of the 1,012 children in these families
589, or 58 per cent., were affected. It is obvious that this is near the
expected percentage in the case of a dominant trait where matings of
affected with normal individuals prevailed. An unfortunate circumstance
about this malady from the eugenic standpoint is the fact that it is
frequently of the presenile form which comes on late in life so that it is
usually impossible to predict whether an individual of marriageable age is
immune or will later become affected.
[Illustration: FIG. 22
Radiograph (Courtesy of Dr. W. B. Helm) showing polydactyly in a child's
hand. For genealogy of this see Fig. 23, p. 110.]
[Illustration: FIG. 23
Chart showing a history of polydactylism through five generations in the
B---- family. The individual whose hand is pictured in Fig. 22, p. 109, is
of the fifth generation. Squares represent males, circles females.]
Another defect of the eye following the course of a dominant in heredity
is a pigmentary degeneration of the retina known as _retinitis
pigmentosa_. Atrophy of the optic nerve is also involved and the final
result is blindness. Still another example frequently cited is that of
hereditary night blindness (_hemeralopia_), a disease in which the
affected person can not see by any but the brightest light. In most
affected families the final outcome is usually total blindness. One of the
most remarkable pedigrees of defects in man ever collected is one of this
disease published by Nettleship. He succeeded in tracing the defect
through nine generations, back to the seventeenth century. The genealogy
includes 2,116 persons. The character behaves as a single dominant in
males, but frequently, though not always, females may be carriers of the
defect in transmissible form though not exhibiting it themselves. That is,
males in which the condition is simplex (Aa) develop the defect but
females of similar simplex constitution (Aa) frequently do not. It
follows, therefore, that normal males of such strains will have normal
offspring but normal females may have affected children.
[Illustration: FIG. 24
Radiograph (Courtesy of Dr. W. B. Helm) showing a partial syndactyly in
each hand of an individual. Some degree of webbing between the more distal
portions of the affected parts is usual.]
[Illustration: FIG. 25
Pedigree of a family with presenile cataract (black symbols); numbers in
circles indicate unaffected individuals (after Davenport).]
=Other Defects Inherited as Dominants.--=Not to go into details other
defects which behave as dominants or modified dominants in human
inheritance may be mentioned. The following list is not complete and it
must be understood that in some cases the statistics are insufficient to
justify us in making anything but a tentative decision. We may thus
enumerate as dominant over normality: _Achondroplasy_ (abnormally short
limbs with normal head and body); _Keratosis_ (thickening of epidermis);
_Epidermolysis_ (excessive formation of blisters); _Hypotrichosis_
(hairless, toothless condition); _Diabetes insipidus_; _Diabetes
mellitus_; ordinary (not Gower's) _muscular atrophy_; _Glaucoma_ (internal
swelling and pressure of eye-ball); displaced lens; _Coloboma_ (open
suture in iris); spottedness of hair-coat; and corneal opacity.
As a final illustration of a serious malady in man which acts as a
dominant in inheritance, let us take _Huntington's chorea_. Ordinary
_chorea_, or St. Vitus' dance, a disorder characterized by involuntary
muscular movements, is commonly though not always confined to children and
usually ends in recovery, but _Huntington's chorea_ appears typically in
middle life and is a much more dangerous malady. Fig. 26, p. 114,
represents the family history of one of five cases which have been studied
by Doctor Lorenz in the Mendota Hospital for the Insane. All charts which
have been platted of this malady show it to be inherited as a dominant.
This means that half of the children of an individual who carried the
malady in the simplex condition, and all the children of one who carries
it in the duplex condition, are probably marked for this terrible end. And
the true horror of it can only be appreciated by one who has seen the last
stages of the malady. The victim once in its grasp gradually becomes
wrecked in mind and body; the muscular twitchings and disorders of
movement continually increase and dementia progresses until at last death
ensues. Fig. 27, p. 115, is another chart showing inheritance of
_Huntington's chorea_. In still a third case at the Mendota Hospital, the
gravity of the situation can be appreciated when one realizes that the
patient is the father of ten children, ranging in age from one to
seventeen and one-half years. The calamitous fact that this disease does
not manifest itself usually until middle life makes it likely that these
children will all reach maturity, marry and in turn probably produce
offspring before the doomed members of the family realize their fate.
[Illustration: FIG. 26
Chart showing descent of _Huntington's chorea_ in the P---- family
(courtesy of Dr. W. F. Lorenz). Squares represent male, circles female;
shaded figures are choreic members of the family; partially shaded
figures, slightly affected or very "nervous" members. The members of the
last generation are for the most part still too young to show their
condition. The cross indicates the individual in the asylum from whom the
record was traced back.]
CASES OF RECESSIVENESS IN MAN
=Recessive Conditions More Difficult to Deal With Because They Are
Frequently Masked.--=Coming now to the question of recessive conditions in
man, we find that defects are more likely to be of recessive than of
dominant type. Apparently normality usually means the presence of normal
determiners and abnormality, the absence of some essential determiner. In
the latter case, a unit-factor has seemingly been lost out in some way in
the germ-plasm, and the product of such germ-plasm is therefore
incomplete. As long as the loss is counterbalanced by the presence of a
single determiner from the other line of ancestry, that is, as long as the
simplex (Aa) condition prevails, the loss may not be in evidence, except
in cases of incomplete dominance (taints, etc.), but any mating which
permits of the production of the nulliplex condition will bring the defect
to expression again.
[Illustration: FIG. 27
Chart showing inheritance of _Huntington's chorea_ in the R---- family
(courtesy of Dr. W. F. Lorenz); 1, 2 have been patients at Mendota
Hospital for the Insane; 3, died of "paralysis"; the fourth or last
generation indicated by the cross, ranging in age from 6 to 14, are too
young yet to show their condition as regards this malady.]
The obscure nature of recessives makes such conditions more difficult to
deal with than dominant defects. For as regards the latter we have seen
that marriage of unaffected members of the family as far as that
particular trait is concerned, is perfectly safe, even to a cousin, for
once the germ-plasm is purged of such a positive factor, it, in so far as
we know, remains pure. But in the case of a recessive character due to the
absence of some necessary determiner a normal offspring of simplex
constitution (Aa) will probably transmit to half of his children the
capacity for handing on the defect, or if mated to another normal
individual of simplex constitution (Aa) is likely to have the actual
defect revealed again in one-fourth of his children and latent in
two-thirds of the remainder.
=Albinism a Recessive.--=As an easily understood illustration of this type
of case we may take human albinism, a condition which is due to the
absence of a pigment-developing determiner. According to Davenport the
albinic condition is recessive to normal condition. If albino (aa) is
mated with albino (aa) nothing but albino children may be expected. An
albino (aa) mated with a normal individual will have normal offspring
(Aa), but they will have the capacity for transmitting albinism to their
descendants. Thus the normal offspring (Aa) of an albino (aa) and a normal
parent (AA) if mated to another normal individual (Aa) who has also had an
albino parent will probably transmit actual albinism to one-fourth of his
children and the same capacity that he himself has of producing albinos,
to one-half of his children, although the latter will appear to the eye to
be normal.
=Other Recessive Conditions in Man.--=If for albinism we substitute
certain forms of insanity, hereditary feeble-mindedness (Fig. 28, p. 118),
or hereditary epilepsy, all of which apparently follow the same law, we
can readily understand how unfit such matings are where both strains are
affected. Marriage with similarly defective stock will result in the
affection appearing in one-fourth of the progeny, and one-half of them,
though apparently normal themselves, will have the capacity for
transmitting the imperfection. It is in the existence of such hidden
factors that the chief danger in the marriage of cousins, or in fact any
consanguineous marriage lies.
A few of the various defects which seem to be inherited as recessives when
mated with normality are: susceptibility to cancer; _chorea_ (St. Vitus'
dance); true dwarfism (all parts proportionately reduced); _Alkaptonuria_
(urine darkens after passage); alcoholism and criminality, where based on
mental deficiency; hereditary _hysteria_; _multiple sclerosis_ (diffuse
degeneration of nervous tissue); _Friedreich's disease_ (degeneration of
upper part of the spinal cord); _Merriere's disease_ (dizziness and
roaring in ears); _Thomsen's disease_ (lack of muscular tone); hereditary
_ataxia_; possibly the tendency to become hard of hearing with increased
age; and possibly, non-resistance to tuberculosis.
Of non-pathological conditions in man which are inherited as recessives,
apparently either very great or very small intellectual ability are
examples.
[Illustration: FIG. 28
Chart showing descent of feeble-mindedness as a typical recessive (after
Goddard). Squares represent males, circles females; DD, homozygous
dominant; DR, heterozygous dominant (i. e. normal although a carrier); RR,
pure recessive; N, normal; F, feeble-minded; A, alcoholic.]
=Breeding Out Defects.--=Even though recessive defects occur in a stock,
there is the possibility of diluting out the imperfection in successive
generations if care is taken always to marry into a stock wholly free from
it. For example, a normal individual carrying a recessive defect will bear
the abnormality in half of his or her germ-cells. This means that when
such an individual marries a normal, non-carrier, half of their children
will be wholly normal (AA) and half will be carriers; normal but of
simplex constitution (Aa). If now this generation, carriers and
non-carriers, marry only into normal strains of duplex constitution, then
their combined issue will be likewise normal with only one-fourth of them
carriers of the imperfections. This means that even if all of this last
generation were married to persons having the defect only one out of four
would have children showing it although the remaining children would be
carriers. On the other hand if mated to normals only one-eight of the next
generation would be carriers. Thus by continually marrying into strong
strains liability to manifest any recessive defect can be diminished in a
few generations until the descendants are no more likely to have defective
children than are members of our ordinary population.
The proportion in which the recessive defect would appear in successive
generations if all persons in a given generation married only normal
individuals who were non-carriers is indicated in the following table
where AA indicates a normal individual, Aa one who is normal but a
carrier, and aa an individual with the imperfection expressed; to indicate
proportions simply after the first generation, four is arbitrarily chosen
as the number of children which results from each marriage:
Matings Children
Generation 1 aa x AA = Aa
Generation 2 Aa x AA = 2AA + 2Aa
Generation 3 AA x AA = 4AA
AA x AA = 4AA
Aa x AA = 2AA + 2Aa
Aa x AA = 2AA + 2Aa
----------
12AA + 4Aa
=Other Inheritable Conditions in Man.--=While many pedigrees show beyond
dispute that such qualities as musical ability, literary ability, memory,
calculating ability, mechanical skill, longevity, peculiarities of
handwriting, obesity and muscular strength, for example, are inherited,
their modes of inheritance have not yet been sufficiently analyzed to
express them exactly.
CHAPTER V
ARE MODIFICATIONS ACQUIRED DIRECTLY BY THE BODY INHERITED?
=Which New Characters Are Inherited?--=Any new feature which appears in a
given organism may have had its origin in some change which has come about
in the germ from which it sprang, or it may be merely the product of some
unusual stimulus operating on the body. While the outcome, as far as the
present individual is concerned, is in each case a definite modification,
the matter of inheritance is a very different question. On the first
alternative where the new character is the outcome of germinal change, it
is obvious that the altered germ-plasm will find expression in a similar
way in succeeding generations as long as the new germinal combinations
persist. On the other hand, if the new character has resulted merely from
some influence operating on the body of the individual, then to be
inherited it would also have in some way to be transferred to and
incorporated in the germ-plasm. Inasmuch as the body or soma of any
individual is highly plastic and since various of its ultimate features
may be mere somatic modifications, it is important to decide if possible
whether or not somatic variations which are not of germinal origin can be
inherited.
=Examples of Somatic Modifications.--=For example, the small foot of the
Chinese woman of certain caste is the result of inherent germinal factor
for the production of a foot plus the effects of binding which are in no
wise germinal. The hand of the skilled pianist is a normal hand of
germinal origin and normal environment plus the effects of special
training. Again, the head of the Flathead Indian is a normal head of
germinal origin and environment plus the effects of flattening. Similarly,
almost any malformation of extrinsic origin may be cited, ranging from
mutilations and amputations, scars and the like to monstrosities such as
one-eyed fish which may be produced by subjecting a developing embryo to
adverse conditions of development.
=Use and Disuse.--=Even reactions set up through the organism's own
activities must produce changes. For example, a muscle has a certain
average of normal development in the average man; it comes to this through
the innate nature of its component cells plus a certain average amount of
exercise. It may, however, be developed far beyond this average by
excessive exercise. On the other hand, it is a well-known fact that an
unused organ weakens or may remain but partially developed. Thus either
use or disuse may play an important part in the molding of a given
individual. But whether or not in doing this it similarly affects the germ
is a very different matter.
=The Problem Stated.--=The question is can such enhanced or suppressed
development, or can new or modified characters, produced in an individual
by external agencies be so reflected on the germ-cell of the individual
that they tend to reappear _as such_ in its offspring without requiring
the same external factors for their production?
=Special Conditions Prevail in Mammals.--=Before proceeding further we
must recognize clearly the very special conditions which exist in most
mammals. With them environment is in part an intra-maternal environment
and in part independent of parental influences. Thus the formula for most
non-mammalia would be--
Individual == egg + non-parental environment; but
for most mammals, including man--
Individual == egg + intra-maternal environment + non-parental
environment.
This condition in mammals introduces a complicating factor which is likely
to obscure the whole issue unless we bear it constantly in mind. In other
words, we must discriminate sharply, in the discussion of inheritance in
man, for instance, between two classes of influences which may exist in
the infant at birth, that is, which are _congenital_; namely, those which
were truly inherent--were in the germ-cells--at the very inception of the
young individual, and (2) those which might later have been derived from
either parent by the yet unborn offspring. The latter are not regarded as
truly hereditary. Since certain diseases or their effects belong here we
occasionally find a physician using the term inheritance for such prenatal
influences, but the more careful ones now employ the term _transmission_
to discriminate between such conditions and true inheritance. In its
biological usage inheritance always refers to germinal constitution and
never to any condition that may be thrust on a developing organism before
birth. It is clear, then, that congenital conditions are not all
necessarily cases of inheritance.
=Three Fundamental Questions.--=To get at the question of the inheritance
of body modifications with the least confusion, let us examine it in the
form of three fundamental questions, as follows:
1. Can external influences directly affect the germ-cells?
2. Can external influences, operating through the intermediation of the
parental body, affect the germ-cells? If so, is the effect a specific and
a permanent one which persists in succeeding generations independently of
external influences similar to those which originally produced it? Only
such a condition as this would rank as the inheritance of a somatic
modification.
3. Can the appearance of new characters be explained on any other ground,
or on any more inclusive basis, than through the transmission of somatic
acquirements, or do organisms possess heritable characters which are
inexplicable as inheritance of such modifications?
Obviously the only way the question can be settled is through careful
experimentation in which all possible sources of error have been foreseen
and guarded against. Much experimental work has been undertaken for the
solution of this problem as the goal and we may therefore select typical
ones of these experiments and apply the results toward answering our three
questions.
=External Influences May Directly Affect the Germ-Cells.--=There is
evidence that under special conditions external influences may in certain
organisms affect the germ-cells, but that this occurs commonly is
extremely doubtful. For example, Professor MacDougal, by treating the
germ-cells of the evening primrose with various solutions, such as sugar,
zinc sulphate and calcium nitrate, has apparently succeeded in producing
definite germinal mutations. He injected the solution into the ovary of
the flower the forenoon of the day at the close of which pollination would
occur. He reports that in this way changes were produced in the germ which
found expression in new and permanent characters.
Professor Tower has experimented for a number of years with various
species of _Leptinotarsa_, the potato beetle. By varying the conditions of
temperature, humidity and atmospheric pressure when females were laying
their eggs, he reports having produced variations in the young which came
from these eggs although the mothers themselves were not changed.
According to Professor Tower slight increase or decrease in these
environmental factors stimulated the activity of the color producing
ferments, giving rise to melanic or darker individuals.
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