Inheritance of Characteristics in Domestic Fowl
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Title: Inheritance of Characteristics in Domestic Fowl
Author: Charles Benedict Davenport
Illustrator: Kako Morita
Kenji Toda
Release date: February 17, 2015 [eBook #48288]
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*** START OF THE PROJECT GUTENBERG EBOOK INHERITANCE OF CHARACTERISTICS IN DOMESTIC FOWL ***
INHERITANCE OF CHARACTERISTICS IN DOMESTIC FOWL.
BY
CHARLES B. DAVENPORT,
DIRECTOR OF THE STATION FOR EXPERIMENTAL EVOLUTION, CARNEGIE
INSTITUTION OF WASHINGTON.
[Illustration]
WASHINGTON, D. C.
PUBLISHED BY THE CARNEGIE INSTITUTION OF WASHINGTON. 1909
CARNEGIE INSTITUTION OF WASHINGTON PUBLICATION NO. 121.
PAPERS OF THE STATION FOR EXPERIMENTAL EVOLUTION, NO. 14.
PRESS OF J. B. LIPPINCOTT COMPANY PHILADELPHIA
TABLE OF CONTENTS.
PAGE
INTRODUCTION 3
CHAPTER I. THE SPLIT OR Y COMB 5
A. Interpretation of the Y Comb 5
B. Variability of the Y Comb and Inheritance of the
Variations 12
CHAPTER II. POLYDACTYLISM 17
A. Types of Polydactylism 17
B. Results of Hybridization 18
CHAPTER III. SYNDACTYLISM 29
A. Statement of Problem 29
B. Results of Hybridization 32
CHAPTER IV. RUMPLESSNESS 37
CHAPTER V. WINGLESSNESS 42
CHAPTER VI. BOOTING 43
A. Types of Booting 43
B. Normal Variability 43
C. Results of Hybridization 46
CHAPTER VII. NOSTRIL-FORM 59
CHAPTER VIII. CREST 67
CHAPTER IX. COMB-LOP 69
CHAPTER X. PLUMAGE COLOR 71
A. The Gametic Composition of the Various Races 71
1. White 71
2. Black 72
3. Buff 72
B. Evidence 72
1. Silkie × Minorca (or Spanish) 72
2. Silkie × White Leghorn 75
3. Silkie × Buff Cochin 76
4. White Leghorn × Black Minorca 77
5. White Leghorn × Buff Cochin 77
6. Black Cochin × Buff Cochin 78
CHAPTER XI. INHERITANCE OF BLUE COLOR, SPANGLING, AND BARRING 79
A. Blue Color 79
B. Spangling 80
C. Barring 81
1. White Cochin × Tosa 81
2. White Leghorn Bantam × Dark Brahma 82
3. White Leghorn Bantam × Black Cochin 82
CHAPTER XII. GENERAL DISCUSSION 85
A. Relation of Heredity and Ontogeny 85
B. Dominance and Recessiveness 88
C. Potency 92
D. Reversion and the Factor Hypothesis 93
E. The Limits of Selection 94
1. Increasing the Red in the Dark Brahma × Minorca
Cross 94
2. Production of a Buff Race by Selection 95
F. Non-inheritable Characters 96
G. The Rôle of Hybridization in Evolution 97
LITERATURE CITED 99
===============================================
INHERITANCE OF CHARACTERISTICS
IN DOMESTIC FOWL.
BY
CHARLES B. DAVENPORT.
===============================================
INTRODUCTION.
A series of studies is here presented bearing on the question of
dominance and its varying potency. Of these studies, that on the Y
comb presents a case where relative dominance varies from perfection
to entire absence, and through all intermediate grades, the average
condition being a 70 per cent dominance of the median element. When
dominance is relatively weak or of only intermediate grade the
second generation of hybrids contains extracted pure dominants in
the expected proportions of 1:2:1; but as the potency of dominance
increases in the parents the proportion of offspring with the
dominant (single) comb increases from 25 per cent to 50 per cent.
This leads to the conclusion that, on the one hand, dominance varies
quantitatively and, on the other, that the degree of dominance is
inheritable.
The studies on polydactylism reveal a similar variation of potency in
dominance and show, in Houdans at least, an inheritance of potency
(table 11), and moreover they suggest a criticism of Castle's
conclusion of inheritance of the degree of polydactylism.
Syndactylism illustrates another step in the series of decreasing
potency of the dominant. On not one of the F1 generation was the
dominant (syndactyl) condition observed; and when these hybrids were
mated together the dominant character appeared not in 75 per cent but
in from 10 per cent to 0 per cent of the offspring. The question may
well be asked: What is then the criterion of dominance? The reply
is elaborated to the effect that, since dominance is due to the
presence of a character and recessiveness to its absence, dominance
may fail to develop, but recessiveness never can do so. Consequently
two extracted recessives mated _inter se_ can not throw the dominant
condition; but two imperfect dominants, even though indistinguishable
from recessives, will throw dominants. On the other hand, owing to
the very fact that the dominant condition often fails of development,
two extracted "pure" dominants will, probably always, throw some
apparent "recessives." Now, two syndactyls have not been found that
fail (in large families) to throw normals, but extracted normals
have been found which, bred _inter se_, throw only normals; hence,
"normal-toe" is recessive. In this character, then, dominance almost
always fails to show itself in the heterozygote and often fails in
pure dominants.
The series of diminishing potency has now brought us to a point
where we can interpret a case of great difficulty, namely, a case of
rumplessness. Here a dominant condition was originally mistaken for
a recessive condition, because it never fully showed itself in F1
and F2. Nevertheless, in related individuals, the condition is fully
dominant. We thus get the notion that a factor that normally tends to
the development of a character may, although present, fail to develop
the character. Dominance is lacking through _impotence_.
The last term of the series is seen in the wingless cock which left
no wingless offspring in the F1 and F2 generations. In comparison
with the results gained with the rumpless cock, winglessness in this
strain is probably dominant but impotent.
When a character, instead of being simply present or absent, is
capable of infinite gradations, inheritance seems often to be
blending and without segregation. Two cases of this sort--booting
and nostril-height--are examined, and by the aid of the principle
of imperfect dominance the apparent blending is shown to follow
the principle of segregation. Booting is controlled by a dominant
inhibiting factor that varies greatly in potency, and nostril-height
is controlled by an inhibiting factor that stops the over-growth of
the nasal flap which produces the narrow nostril.
The extracted dominants show great variability in their progeny, but
the extracted recessives show practically none. This is because a
positive character may fail to develop; but an absent character can
not develop even a little way. The difference in variability of the
offspring of two extracted recessives and two extracted dominants
is the best criterion by which they may be distinguished, or by
which the presence (as opposed to the absence) of a factor may be
determined.
The crest of fowl receives especial attention as an example of a
character previously regarded as simple but now known to comprise two
and probably more factors--a factor for erectness, one for growth,
and probably one or more that determine the restriction or extension
of the crested area.
The direction of lop of the single comb is an interesting example
of a character that seems to be undetermined by heredity. In this
it agrees with numerous right and left handed characters. It is not
improbable that the character is determined by a _complex_ of causes,
so that many independent factors are involved.
A series of studies is presented on the inheritance of plumage color.
It is shown that each type of bird has a gametic formula that is
constant for the type and which can be used with success to predict
the outcome of particular combinations. New combinations of color
and "reversions" receive an easy explanation by the use of these
factors. The cases of blue, spangled, and barred fowl are shown also
to contain mottling or spangling factors.
CHAPTER I.
THE SPLIT OR Y COMB.
A. INTERPRETATION OF THE Y COMB.
When a bird with a single comb, which may be conveniently symbolized
as I, is crossed with a bird with a "V" comb such as is seen in
the Polish race, and may be symbolized as oo, the product is a
split or Y comb. This Y comb is a _new form_. As we do not expect
new forms to appear in hybridization, the question arises, How is
this Y comb to be interpreted? Three interpretations seem possible.
According to one, the antagonistic characters (allelomorphs) are I
comb and oo comb, and in the product neither is recessive, but both
dominant. The result is a case of particulate inheritance--the single
comb being inherited anteriorly and the oo comb posteriorly. On
this interpretation the result is not at all Mendelian.
According to the second interpretation the hereditary units are
not what appear on the surface, but each type of comb contains two
factors, of which (in each case) one is positive and the other
negative. In the case of the I comb the factors are presence of
median element and absence of lateral or paired element; and in the
case of the oo comb the factors are absence of median element and
presence of lateral element. On this hypothesis the two positive
factors are dominant and the two negative factors are recessive.
The third hypothesis is intermediate between the others. According
to it the germ-cells of the single-combed bird contain a median unit
character which is absent in the germ-cells of the Polish or Houdan
fowl. This hypothesis supposes further that the absence of the median
element is accompanied by a fluctuating quantity of lateral cere, the
so-called V comb.
The split comb is obtained whenever the oo comb is crossed with a
type containing the median element. Thus, the offspring of a oo
comb and a pea comb is a split pea comb, and the offspring of a
oo comb and a rose comb is a split rose. The three hypotheses may
consequently be tested in three cases where a split comb is produced.
TABLE 1.
+-----------------------+-----+-----+------------+
| | I | Y | No median. |
+-----------------------+-----+-----+------------+
| I × I | 100 | 0 | 0 |
| I × Y | 50 | 50 | 0 |
| I × no median | 0 | 100 | 0 |
| Y × no median | 0 | 50 | 50 |
| No median × no median | 0 | 0 | 100 |
+-----------------------+-----+-----+------------+
The first and third hypotheses will give the same statistical result,
namely, the products of two Y-combed individuals of F1 used as
parents, will exhibit the following proportions: median element, 25
per cent; split comb, 50 per cent; and no median element, 25 per
cent. These proportions will show themselves, whatever the generation
to which the Y-combed parents belong, whether both are of
generation F1, or F2, or F3, or one parent of one generation and the
other of another. Other combinations of parental characters should
give the proportions in the progeny shown in table 1.
On the second hypothesis, on the other hand, the proportions of
the different kinds occurring in the progeny will vary with the
generation of the parents. This hypothesis assumes the existence in
each germ-cell of the original parent of two comb allelomorphs, _M_
and _l_ in single-combed birds and _m_ and _L_ in the Polish fowl,
the capital letter standing for the presence of a character (Median
element or Lateral element) and the small letter for the absence of
that character. Consequently, after mating, the zygote of F1 contains
all 4 factors, _MmLl_, and the soma has a Y comb; but in the
germ-cells, which contain each only 2 unlike factors, these factors
occur in the following 4 combinations, so that there are now 4 kinds
of germ-cells instead of the 2 with which we started. These are _ML_,
_Ml_, _mL_, and _ml_. Furthermore, since in promiscuous mating of
birds these germ-cells unite in pairs in a wholly random fashion, 16
combinations are possible, giving 16 F2 zygotes (not all different)
as shown in table 2.
TABLE 2.
+-------+---------------+---------+
| Type. | Zygotic | Soma. |
| | constitution. | |
+-------+---------------+---------+
| _a_ | M2L2[A] | Y |
+-------+---------------+---------+
| _b_ | M2Ll | Y |
+-------+---------------+---------+
| _c_ | MmL2 | Y |
+-------+---------------+---------+
| _d_ | MmLl | Y |
+-------+---------------+---------+
| _e_ | M2Ll | Y |
+-------+---------------+---------+
| _f_ | M2l2 | I |
+-------+---------------+---------+
| _g_ | MmLl | Y |
+-------+---------------+---------+
| _h_ | Mml2 | I |
+-------+---------------+---------+
| _i_ | mLML | Y |
+-------+---------------+---------+
| _k_ | mLMl | Y |
+-------+---------------+---------+
| _l_ | m2L2 | oo |
+-------+---------------+---------+
| _m_ | m2Ll | oo |
+-------+---------------+---------+
| _n_ | mlML | Y |
+-------+---------------+---------+
| _o_ | mlMl | I |
+-------+---------------+---------+
| _p_ | m2Ll | oo |
+-------+---------------+---------+
| _q_ | m2l2 | Absent |
+-------+---------------+---------+
[A] This convenient form of zygotic formulæ, using a
subscript 2 instead of doubling the letter, is proposed
by Prof. W. E. Castle.
It is a consequence of this second hypothesis that, in F2, of every
16 young 9 should have the Y comb; 3 the I comb; 3 the oo comb,
and 1 no comb at all. It follows further that the progeny of two F2
parents will differ in different families. Thus if a Y-combed bird
of type _a_ be mated with a bird of any type, _all_ of the progeny
will have the Y comb.
From Y-combed parents of various types taken at random 4 kinds of
families will arise having the following percentage distribution of
the different types of comb:
1. Y comb, 100 per cent.
2. Y comb, 75 per cent; I comb, 25 per cent.
3. Y comb, 75 per cent; oo comb, 25 per cent.
4. Y comb, 56.25 per cent; I comb, 18.75 per cent; oo comb,
18.75 per cent; absent, 6.25 per cent.
Again, mating two extracted I combs of F2 should yield, in F3, two
types of families in equal frequency as follows:
1. I comb, 100 per cent.
2. I comb, 75 per cent; no comb, 25 per cent.
Again, mating two extracted oo combs of F2 should yield, in F3, two
types of families in equal frequency, as follows:
1. oo comb, 100 per cent.
2. oo comb, 75 per cent; no comb, 25 per cent.
Single comb × Y comb should give families of the types:
1. Y comb, 100 per cent.
2. Y comb, 50 per cent; I comb, 50 per cent.
3. Y comb, 50 per cent; oo comb, 50 per cent.
4. Y comb, 25 per cent; I comb, 25 per cent; oo comb, 25
per cent; absent, 25 per cent.
Mating oo comb and Y comb should give the family types:
1. Y comb, 100 per cent.
2. Y comb, 50 per cent; oo comb, 50 per cent.
3. Y comb, 50 per cent; I comb, 50 per cent.
4. Y comb, 25 per cent; oo comb, 25 per cent; I comb, 25
per cent; no comb, 25 per cent.
Finally, I comb and oo comb should give the following types of
families:
1. Y comb, 100 per cent.
2. I comb, 100 per cent.
3. Y comb, 50 per cent; oo comb, 50 per cent.
4. I comb, 50 per cent; no comb, 50 per cent.
Now, what do the facts say as to the relative value of these three
hypotheses? Abundant statistics give a clear answer. In the first
place, the progeny of two Y-combed F1 parents is found to show the
following distribution of comb types: Y comb 471, or 47.3 per cent;
I comb 289, or 29.0 per cent; oo comb 226, or 22.7 per cent; and
no comb 10, or 1 per cent. The presence of no comb in F2 speaks for
the second hypothesis, but instead of the 6.25 per cent combless
expected on that hypothesis only 1 per cent appears. There is no
close accord with expectation on the second hypothesis.
Coming now to the F3 progeny of two Y-combed parents, we get the
distribution of families shown in table 3.
TABLE 3.
+--------+-------------------------+---------------------------+
| Pen No.| Parents. | Comb in offspring. |
| |-------------------------+------+-----+-----+--------|
| | ♀ (F2) | ♂ (F2) | I | Y | oo | Absent.|
|--------+-------------+-----------+------+-----+-----+--------+
| 707 | { 366 | 1378 | 18 | 16 | 9 | .... |
| | { 522 | 1378 | 1 | 1 | 0 | .... |
| | | | | | | |
| | { 2250 | 2247 | 9 | 5 | 4 | 1 |
| 763 | { 2700 | 2247 | 3 | 5 | 3 | 1 |
| | { 3799 | 2247 | 5 | 4 | 3 | .... |
| | | | | | | |
| 769 | { 1305 | 911 | 7 | 4 | 6 | .... |
| | { 2254 | 911 | 15 | 15 | 7 | .... |
| |------+-----+-----+--------+
| Totals (142) | 58 | 50 | 32 | 2 |
| Proportions (per cent) | 40.8 | 35.2| 22.5| 1.4 |
| | | | ⎿⎵⎵⎵⏌ |
| | | | 23.9 |
+---------+-------------+----------+------+-----+--------------+
An examination of these families shows not one composed exclusively
of Y-combed individuals nor those (of significant size) containing
Y-combed and I-combed or oo-combed individuals exclusively,
much less in the precise proportion of 3:1, yet such should
be the commonest families if the second hypothesis were true.
Notwithstanding the marked deviation--to be discussed later--from
the expected proportions of I, 25 per cent; Y, 50 per cent;
oo, 25 per cent, the result accords better with the first or third
hypothesis. Since on either of these hypotheses the same proportions
of the various types of comb are to be expected in the progeny of
Y-combed parents of whatever generation, it is worth recording that
from such parents belonging to all generations except the first the
results given in table 4 were obtained, and it will be noticed that
these results approach expectation on the first or third hypothesis.
TABLE 4.
+-----------+-------+-------+-------+---------+--------+
| | I | Y | oo | Absent. | Total. |
+-----------+-------+-------+-------+---------+--------+
|Frequency | 235 | 291 | 144 | 12 | 682 |
| | | | | | |
|Percentage | 34.5 | 42.7 | 21.1 | 1.8 | .... |
+-----------+-------+-------+-------+---------+--------+
The progeny of two extracted single-combed parents of the F2
generation give in 3 families the following totals: Of 95 F3
offspring, 94 have single combs; one was recorded from an unhatched
chick as having a _slightly_ split comb, but this was probably a
single comb with a slight side-spur, a form that is associated with
purely I-combed germ-cells. This result is in perfect accord with
the second and third hypotheses, but is irreconcilable with the first
hypothesis.
The progeny of two extracted oo-combed parents is given in table 5.
TABLE 5.
+-----+-----------------+----------------------+
| | Parents. | Comb in offspring. |
| Pen +--------+--------+----+----+----+-------+
| No. | ♀ | ♂ | I | Y | oo |Absent.|
| | (F2). | (F2). | | | | |
+-----+--------+--------+----+----+----+-------+
| 729 |{2255 | 936 | .. |[A]4| 36| .. |
| |{2269 | 936 | .. | .. | 29| .. |
| | | | | | | |
| |{ 369 | 1390 | 1 | .. | 3| .. |
| 756 |{1067 | 1390 | .. | .. | 8| 1 |
| |{1113 | 1390 | .. | .. | 13| 4 |
| | | | | | | .. |
| |{2011 | 444 | .. | .. | 10| .. |
| |{2011 | 2621 | .. | .. | 9| .. |
| |{2333 | 444 | .. |[A]5| 11| .. |
| |{2333 | 2621 | .. |[A]1| 2| .. |
| 762 |{2618 | 444 | .. | .. | 2| .. |
| |{2618 | 2621 | .. | .. | 5| .. |
| |{3776 | 444 | .. | .. | 2| .. |
| |{3776 | 2621 | .. | 1 | 14| .. |
| | | | | | | |
| |{2016 | 4731 | .. | .. | 10| .. |
| 820 |{2255 | 4731 | .. | .. | 16| .. |
| |{5143 | 4731 | .. | .. | 45| .. |
| |{6479 | 4731 | .. | .. | 31| .. |
| | | | | | | |
| |{[B]2618| 5119 |[B]1| .. | 23| .. |
| |{3776 | 5119 | .. | .. | 28| .. |
| 832 |{4404 | 5119 | .. | .. | 9| .. |
| |{4732 | 5119 | .. | .. | 3| .. |
| |{5803 | 5119 | .. | .. | 21| 2 |
| |{6481 | 5119 | .. | .. | 11| .. |
| | | | | | | |
| 834 | 2324 | 5090 | .. | .. | 26| .. |
| +----+----+---+--------+
| Total | 2 | 11 | 367| 7 |
+--------------+--------+----+----+------------+
[A] Median element recorded as "small" in these offspring.
[B] A median element visible in the mother, No. 2618.
The distribution of offspring in the 24 families of table 5 is
in fair accord with any of the three hypotheses, but seems to
favor the second, for that hypothesis calls for families with
combless children, whereas such are not to be expected on the first
hypothesis. Moreover, agreement with the second hypothesis is fairly
close, for that calls for 3 families with combless children and
there were actually 3 such families showing a total of 1.8 per cent
combless, where expectation is 2.8 per cent. What is opposed to
any hypothesis is the appearance of some Y-combed offspring; and
to account for this the hypothesis is suggested that the germ-cells
of some parents with oo comb contain traces of the I-comb
determiner. The word "traces" is used because the median element in
these Y-combed offspring is practically always very small. It is
fair, consequently, to conclude that oo × oo gives oo-combed,
and occasionally combless, offspring. This conclusion is further
supported by the statistics derived from extracted oo comb of _all_
generations bred _inter se_, which give: Y 11, oo 427, and no
comb 8, where the 11 Y-combed birds are those just referred to as
progeny of F2 parents. The non-median comb, consequently, probably
contains only non-median germ-cells.
TABLE 6.
+---+-------------------------------------------------+--------------+
| | Parents. | Offspring. |
| |--------+-----+----------+------+-----+----------+----+----+----+
|Pen| ♀ |Form | Degree | ♂ |Form | Degree | | | |
|No.| (F2). | of | of | (F2).| of | of | I | Y | oo |
| | |comb.|splitting.| |comb.|splitting.| | | |
|---|--------|-----|----------|------|-----|----------|----|----|----|
| | | | _P. ct._ | | | _P. ct._ | | | |
| | { 427 | Y | 5 | 439 | I | 0 | 5 | 1 | .. |
|628| { 722 | Y | 20 | 439 | I | 0 | 1 | 5 | .. |
| | { 725 | Y | 10 | 439 | I | 0 | 5 | 3 | .. |
| | | | | | | | | | |
|629| 427 | I | 0 | 491 | Y | 50 | 9 | 6 | .. |
| | | | | | | | | | |
|765| 1790 | I | 0 | 1794 | Y | 90 | 17 | 25 | .. |
| | | | | | | | | | |
| | {3846 | I | 0 | 6652 | Y | 90 | 8 | 5 | .. |
|802| {5025 | I | 0 | 6652 | Y | 90 | 14 | 11 | 2 |
| | {5087 | I | 0 | 6652 | Y | 90 | 13 | 17 | 2 |
| | | | | | | | | | |
|812| {4254 | I | 0 | 4118 | Y | 90 | 15 | 13 | .. |
| | {5540 | I | 0 | 4118 | Y | 90 | 8 | 9 | .. |
| |----|----|----|
| Totals (189) | 95 | 95 | 4 |
| Percentages |49.0|49.0| 2.0|
+-----------------------------------------------------+----+----+----+
The mating of extracted I comb and Y comb, both of the second (or
later) hybrid generation, gives the following distribution of types
in the offspring (table 6): Y comb 95 (49 per cent); I comb 95
(49 per cent); oo comb 4 (2 per cent). In detail the results given
in table 6 accord badly with the second hypothesis, which demands
some families with 100 per cent Y comb.
The mating of extracted oo comb×Y comb, where both parents are of
the second hybrid generation, gave the distribution of comb types in
the 6 families that are recorded in table 7.
TABLE 7.
+---+-----------------+-------------------------+
| | Parents. | Offspring. |
|Pen|-----------------+-----+-----+-----+-------|
|No.| ♀ | ♂ | I | Y | oo |Absent.|
| | (F2). | (F2). | | | | |
+------------+--------|-----+-----|-----|-------|
|634| { 298 | 444 | 0 | 15 | 18 | .... |
| | { 366 | 444 | 5 | 23 | 15 | .... |
| | | | | | | |
|729| { 913 | 936 | 2 | 28 | 37 | .... |
| | { 935 | 936 | ....| 13 | 39 | .... |
| | | | | | | |
|756| { 1043 | 1390 | ....| 13 | 11 | 1 |
| | { 1048 | 1390 | ....| 0 | 5 | .... |
| |-----+-----+-----+-------+
| Totals (214) | 7 | 92 | 115 | 1 |
+---------------------+-----+-----+-----+-------+
The single comb recorded in the case of 7 birds is doubtless merely
the limiting condition of a Y comb in which the median element is
developed to its fullest extent. All but 2 of the 7 were recorded
from early embryos when an incipient bifurcation would be more
difficult to detect. This explanation applies generally, and accounts
for the usual excess of I comb when compared with Y comb, as
for instance in table 3, page 7. Returning to table 7, it is,
consequently, probable that only the Y-combed and non-median-combed
offspring are produced and that they are in the proportion of 99 to
115 or of 46 per cent to 54 per cent. If we add together all records
of a oo×Y cross, disregarding the generation of the parents, we
get a total I comb 5,[1] Y comb 177, oo comb 172, and absent 3,
or 182 (51 per cent) with the median element and 175 (49 per cent)
without. Thus the oo×Y cross gives the 1:1 proportion called for
on the first and third hypotheses and not at all the variety required
by the second hypothesis.
[1] Excluding 6 doubtful because from too young embryos
and not observed by myself.
TABLE 8.
+---+----------------------+---------------+------------------+
| | Mother. | Father. |Comb in offspring.|
|Pen+---------+-----+------+--------+------+---+---+----+-----+
|No.| | |P. ct.| | | | | | |
| | No. |Comb.|split.| No. |Comb. | I | Y | oo | Abs.|
+---+---------+-----+------+--------+------+---+---+----+-----+
|704| { 65 F1| Y | 50 | 1420 F2|Absent|.. | 10| 6 | 8 |
| | {1061 F2| Y | 50 | 1420 F2| Do. |.. | 4| .. | 1 |
| | | | | | | | | | |
|819| { 57 F1| Y | 50 | 1420 F2| Do. |.. | 8| 6 | 5 |
| | { 65 F1| Y | 60 | 1420 F2| Do. |.. | 1| .. | 1 |
| +---+---+----+-----+
| Total | 0 | 23| 12 | 15 |
+------------------------------------------+---+---+----+-----+
Finally, we must consider the result of mating a bird without
papillæ (No. 1420, pen 704) with a median-combed hen (480). When
this typical single-combed hen was used the 49 progeny were all of
the Y type.[2] This proves that the combless type behaves only as
an extreme of the non-median type.
[2] One is reported as having a I comb; probably the limiting
condition, again.
When Y-combed hens were used with the combless cock the offspring
had Y comb and non-median-comb in nearly equal numbers, 23:27
(table 8), but the latter included an unusually large proportion
of combless fowl (15 in 27). When a combless hen (No. 4257) was
used, 9 of the offspring had oo comb and 2 no comb; not a greater
proportion of combless birds than in the no-comb×Y-combed cross.
All of these facts indicate that "comblessness" is not entire absence
of the comb factors, but a minimum case of the oo or paired comb.
This result is opposed to the second hypothesis.
The statistics of all matings between I, Y, and no comb on the
one side and no comb on the other thus speak unanimously for the
conclusion that in these matings we are not dealing with 2 pairs
of allelomorphs, but with a single comb and its absence (third
hypothesis) or with a case of particulate inheritance (first
hypothesis). Moreover, it must be said that the split comb is
obtained also when the Polish-Houdan comb is crossed with a pea
comb or a rose comb; and the pea and rose combs can not be said to
have "lateral comb absent," as required by the second hypothesis.
Consequently the second hypothesis is definitely excluded.
It now remains to decide between the two remaining hypotheses. First
of all, it may be said that the perfection with which I and oo
combs can be extracted from Y-combed birds indicates that we are
here dealing with a case of Mendelian inheritance and, in so far,
favors the third hypothesis. To accord with the theory of particulate
inheritance, of which the first hypothesis is a special case, the two
united characters should transmit the mosaic purely; but this they do
not do. Hence the third hypothesis is to be preferred to the first.
Comblessness is a necessary consequence of the second hypothesis and
is inexplicable on the first hypothesis. On the third hypothesis
it may be accounted for as follows: Absence of single comb is
allelomorphic to its presence. The lateral comb is a character common
to fowl either with or without the median comb, but it is ordinarily
repressed in the birds with single comb and gains a large size when
the median element is absent. It is a very variable element. At one
extreme it forms the cup comb; at the other there is an absence of
any trace of comb. My own records show all grades between these
extremes, including minute papillæ on both sides of the head or on
one side only, low paired ridges, the butterfly comb, and cup comb
shorter than normal. This variability of the lateral element is
comparable to the fluctuation in size of the single comb itself,
as illustrated by the Single-comb Minorca on the one hand and the
Cochin on the other. It is comparable, also, to the fluctuation in
the paired part of the Y comb, which we shall consider in the next
section, and to the variability of the oo comb as met with in the
pens of fanciers.
The foregoing considerations do not, at first sight, account for the
Y comb as seen in F1. Yet they provide us with all the data for an
explanation. Median comb of the Minorca dominates over no median of
the Polish, and so in F1 we have the median element represented. But,
on the well-known principle of imperfection of dominance in F1, the
median comb is usually incomplete and, probably for some ontogenetic
reason, incomplete only behind. The incompleteness behind permits the
development there of the elsewhere repressed lateral comb, and we
therefore have the Y comb--evidence at the same time of a repressed
lateral-comb Anlage in the single-combed birds and of imperfection of
dominance of the single comb in the first hybrid generation.
B. VARIABILITY OF THE Y COMB AND INHERITANCE OF THE VARIATIONS.
[Illustration: FIG A.--The frequency of the different forms of Y
comb, each form being based on the percentage of the median element
of the Y comb to the entire length of comb.]
As already stated, the proportions of the median and the lateral
elements in the Y comb are very variable; the median element may,
indeed, constitute anywhere from 100 per cent to 0 per cent of the
entire comb. Even full brothers and sisters show this variability.
Thus the offspring of No. 13 ♀ Single-comb Minorca and No.
3 ♂ Polish have the median element of the Y comb ranging
from 0 per cent to 70 per cent of the whole comb. Notwithstanding
this variability of the median element in any family there is a
difference in the average and the range of variability in families
where different races are employed. Thus the offspring of two Polish
× Minorca crosses show an _average_ of 46 per cent of the median
element in the comb; the Houdan × Minorca cross gives combs with 60
per cent of the median element; and in the combs of the offspring of
two Houdan × White Leghorn crosses there is, on the average, 71 per
cent of the median element. The Houdan × Dark Brahma (pea comb) gives
combs with an average of 87 per cent median element and the Polish
× Rose-comb Minorca cross gives 89 per cent median. The rose-combed
hens used in this last cross were heterozygous, having single comb
recessive; consequently they produced also chicks with typical Y
combs. Such had, on the average, only 59 per cent of the median
element and were thus in striking contrast with the slightly split
rose combs. In the case of the partially split rose combs the median
element ranged from 60 per cent to 100 per cent of the whole length
of the comb; but in the split single combs the range is from 0 to
100 per cent. Thus, in the two cases, the proportion of the median
element and the range of its variability differ greatly.
Also, in generations subsequent to the first, the Y comb exhibits
this same variability. We have already seen that the progeny of the
Y-combed offspring of any generation may be compared with those
of any other, and so we may mass together the progeny of all hybrid
generations so long as they are derived from the same ancestral pure
races.
In inquiring into the meaning of this variability we must first
construct the polygon of frequency of the various grades of median
element. This is plotted in fig. A, which is a composite whose
elements are, however, quite like the total curve. There is one
empirical mode at 70 per cent and another at 0 per cent. The smaller
mode at 50 per cent is, I suspect, due to the tendency to estimate
in round numbers, and may be, in this discussion, neglected. From
this polygon we draw the conclusions, first, that the median element
in the Y comb tends to dominate strongly over the absence of this
element, as 7:3, and, second, that dominance is rarely complete. Yet
there is an important number of cases, even in F1, where the median
element is almost or completely repressed (down to 10 to 0 per cent
of the whole) and the comb consists of two high and long lateral
elements--the "cup comb" of Darwin. There are, then, in the offspring
of a median-combed and a non-median-combed parent, two types with few
intergrades--the type of slightly incomplete dominance of the median
element and the type of very incomplete dominance.
We have now to consider how these two types of comb and their
fluctuations behave in heredity. When two parents having each combs
of the 70 per cent or 80 per cent median type are mated, their
offspring belong to the three categories of I, Y, and "no-median"
comb, but the relative frequency of these three categories is not
close to the ideal of 25 per cent, 50 per cent, and 25 per cent,
respectively. For there is actually in 336 offspring a marked
excess of the I comb, 36 per cent, 44 per cent, and 20 per cent,
respectively, resulting. When, on the other hand, two parents having
each combs of the 10 per cent and 0 per cent types are mated their
offspring are of the same three categories and the proportions
actually found in 241 offspring (28 per cent, 47 per cent, 25 per
cent) closely approximate the ideal. It is clear, then, that even the
cup comb, without visible median element, has such an element in its
germ-cells and is totally different in its hereditary behavior from
the Polish comb, in which the median element is absent, not only from
the soma, but also from the germ-cells.
We have seen in the last paragraph that the Y comb with only 10
per cent to 0 per cent median element has germ-cells bearing median
comb as truly as the Y comb containing 70 per cent to 80 per
cent median element, but we have also seen that in the latter case
there is an excess of single-combed progeny. We have now to inquire
whether, in general, there is a close relation between the proportion
of median element in the comb of the parents and the percentage of
single-combed offspring. These relations are brought out in the lower
half of table 9.
TABLE 9.--_Frequency of the different proportions of single
element in the combs of offspring of parents having the average
proportion of median element given in the column at the left._
+-------------+--------------------------------------------------------+
| | Y combs. |
| +--------------------------------------------------------+
| | Offspring. |
| +----+----+----+----+----+----+----+----+----+----+------+
| | 0 | 10 | 20 | 30 | 40 | 50 | 60 | 70 | 80 | 90 |Total.|
+-------------+----+----+----+----+----+----+----+----+----+----+------+
| { 0 | 21 | 5 | 4 | 3 | 4 | 6 | 5 | 10 | 8 | 1 | 67 |
| {10 | 21 | 5 | 3 | 0 | 3 | 9 | 2 | 4 | 2 | 0 | 49 |
| {20 | 5 | 4 | 2 | 1 | 0 | 4 | 2 | 12 | 0 | 1 | 31 |
| {30 | 8 | 17 | 8 | 10 | 9 | 22 | 12 | 30 | 8 | 3 | 127 |
| Parents {40 | 9 | 7 | 4 | 2 | 7 | 39 | 18 | 46 | 26 | 5 | 163 |
| {50 | 7 | 5 | 2 | 1 | 5 | 32 | 13 | 48 | 35 | 11 | 159 |
| {60 | 10 | 7 | 2 | 2 | 2 | 19 | 14 | 47 | 51 | 15 | 169 |
| {70 | 9 | 2 | 4 | 0 | 1 | 6 | 7 | 28 | 41 | 11 | 109 |
| {80 | .. | .. | 1 | 1 | 1 | 1 | 6 | 12 | 11 | 6 | 39 |
| {90 | .. | 2 | 1 | 0 | 0 | 3 | 0 | 3 | 8 | 9 | 26 |
| +----+----+----+----+----|----+----+----+----+----+------+
| Total | 90 | 54 | 31 | 20 | 32 |141 | 79 |240 |190 | 62 | 939 |
+======================================================================+
| | All types of combs in offspring. |
| +----------+---------------------------------------------+
| |Number | I | Y | Non-median. |
| |of +------+--------+------+--------+-----+-------+
| |offspring.| No. | P. ct. | No. | P. ct. | No. | P. ct.|
|-------------+----------+------+--------+------+--------+-----+-------+
| { 0 | 146 | 42 | 20 | 67 | 46 | 37 | 25 |
| {10 | 99 | 25 | 25 | 49 | 50 | 25 | 25 |
| {20 | 73 | 22 | 30 | 31 | 43 | 20 | 27 |
| {30 | 249 | 61 | 25 | 127 | 51 | 61 | 24 |
| Parents {40 | 309 | 73 | 24 | 163 | 53 | 73 | 23 |
| {50 | 329 | 93 | 28 | 159 | 48 | 77 | 23 |
| {60 | 368 | 120 | 33 | 169 | 46 | 79 | 21 |
| {70 | 232 | 80 | 35 | 109 | 47 | 43 | 18 |
| {80 | 104 | 42 | 40 | 39 | 38 | 23 | 22 |
| {90 | 75 | 38 | 51 | 26 | 34 | 11 | 15 |
| +----------+------+--------+------+--------+-----+-------+
| Total | 1984 | 596 | 30.0 | 939 | 47.3 | 449 | 22.7|
+-------------+----------+------+--------+------+--------+-----+-------+
The proportion of single-combed offspring in the total filial
population is 30.0 per cent, a departure of such magnitude from the
expected 25 per cent as to arrest our attention. Further inspection
of table 9 shows that the excess of single-combed offspring is found
only in the lower half of the series. When the percentage of median
element in the parents is under 50 the proportions of I, Y, and
no-median combs are as 25.5 per cent, 49.8 per cent, 24.7 per cent,
or close to expectation; but when the percentage is 50 or over the
proportions are, on the average, 33.6 per cent, 45.2 per cent, and
21.2 per cent, a wide departure from expectation, 1108 individuals
being involved. An examination of table 9 shows, moreover, that
the proportion of offspring with single comb rises steadily as the
proportion of the median element in the parentage increases from 50
per cent. The meaning of this fact is at present obscure, but the
suspicion is awakened that, while the "cup comb" and the more deeply
split combs are typical heterozygotes the slightly split combs are
a complex of 2 or more units, one of which is "single comb." But
that this is not the explanation follows for two reasons: first, that
even in the F1 generation slightly split combs are obtained, and,
second, that the offspring of the cup combs are much more variable
than those of slightly split combs (70 to 90 per cent median). What
is strikingly true is that, from 50 per cent up, as the proportion
of the median element in the parents increases the percentage of
single-combed offspring rises.
The matter may be looked at in another light. Median comb is dominant
over its absence. Typically, we should expect F1 to show a single
comb; the Y comb that we actually get is a heterozygous condition
due to the failure of the median comb to dominate completely.
Typically we should expect F2 to reveal 75 per cent single combs,
of which 1 in 3 is homozygous and 2 in 3 are heterozygous. Owing
to the failure of single comb always to dominate completely in the
heterozygotes, we expect to find some of the 75 per cent with the
Y comb. When in the parents dominance has been very incomplete in
the heterozygote (as is the case in the 0 per cent to 40 per cent
median-combed parents) we find it so in the offspring also and all
heterozygotes show a Y comb of some type. But when in the parents
dominance has been strong in the heterozygote (50 per cent to 90
per cent) it is so in the offspring also and only a part of the
heterozygotes show the Y comb; the others show the single comb and
thus swell the numbers of the single-combed type. The only objection
to this explanation is found in the reduction in the percentages
of the no-median type. Thus, adding together the homozygous and
heterozygous median-combed offspring and comparing with the
non-median-combed, we find these ratios:
Parental per cent 0-40 50 60 70 80 90
Ratio 75.3:24.7 76:23 79:21 82:18 78:22 85:15
There is a great deviation from 25 per cent in the "non-median"
offspring of the 90 per cent parents, but in this particular case
the total number of offspring is not large, and the deviation has a
greater chance of being accidental. Altogether this explanation of
the varying per cents of single comb on the ground of inheritance of
varying potency in dominance seems best to fit the facts of the case.
From the foregoing facts and considerations we may conclude that the
Y comb represents imperfect dominance of median over no-median
comb; that there is a fluctuation in the potency of the dominance, so
that the proportion of the median element varies from 0 to over 90
per cent; that the more potent the dominance of median element is in
any parents the more complete will be the dominance in the offspring
and the smaller will be the percentage of imperfectly dominant, or
Y-combed, offspring. _Dominance varies quantitatively and the
degree of dominance is inheritable._
The index of heredity may be readily obtained in the familiar
biometric fashion from table 9. This I have calculated and found
to be 0.301± 0.002. This agrees with Pearson's theoretical
coefficient of correlation between offspring and parent. The index
is larger than it would otherwise be because it is measured with
an _average_ of the parents and these parents assortatively mated.
But this instance is, in any case, an interesting example of strong
inheritance of a quantitative variation.
What, it may be asked, is the relation of these facts to the general
principle that inheritance is through the gametes? Why, when a gamete
with the median element unites with a gamete without that element,
does the zygote develop a soma that in some cases shows a nine-tenths
median and sometimes a one-tenth median element? We have seen that
the Y comb is a heterozygous form due to imperfection of dominance
of the median element; but why this variation in the perfection of
the median element? This is probably a piece of the question, why
any dominance at all. We find, in general, that the determiner of a
well-developed organ dominates in the zygote over the determiner of a
slightly developed condition of that organ or its obsolete condition.
It is as though there were in the zygote an interaction between the
strong and the weak form of the determiner, and the strong won; but
sometimes the victory is imperfect. In the specific case of comb the
interaction between median and no-median leads to a modification,
weakening, or imperfection of the median element, and this weakening
varies in degree. Sometimes the weakening is inappreciable--when the
comb is essentially single; sometimes it is great, and the result is
a comb in which the median element is reduced to one-half; sometimes,
finally, the determiner of median comb is so completely weakened
by its dilution with "no-median" as not to be able to develop,
and we have the cup comb with only a trace of the median element.
Nevertheless, such a cup comb is heterozygous and produces both
single-combed and Polish-combed germ-cells. Thus the variation in the
extent of the median comb seems to point to variations in relative
potency of the median comb over its absence.
CHAPTER II.
POLYDACTYLISM.
The possession of extra toes is a character that crops out again and
again among the higher, typically 5-toed vertebrates. Many cases have
been cited in works on human and mammalian teratology (_cf._ Bateson,
1904, and Schwalbe, 1906), and it is recognized that this abnormality
is very strongly inherited in man. Bateson and Saunders, and Punnett
(1902 and 1905), Hurst (1905), and Barfurth (1908), as well as myself
in my earlier report, have demonstrated the inheritableness of the
character in poultry. Bateson and Punnett (1905, p. 114) say: "The
normal foot, though commonly recessive, may sometimes _dominate_ over
the extra-toe character, and this heterozygote may give equality
when bred with recessives, just as if it were an ordinary DR."
Altogether, the inheritance of extra-toe diverges so far from typical
Mendelian results as to deserve further study.
A. TYPES OF POLYDACTYLISM.
There are two main types of polydactylism: that in which the inner
toe (I) of the normal foot is replaced by 2 simple toes, and
that in which it is replaced by two toes, of which the mediad is
simple and the laterad is divided distally. The former type is
characteristic of the Houdans; the latter is usually associated with
the Silkies. Both conditions are, however, found in both races. The
simplest condition is seen in many Houdans of my strain. It consists
of 2 equal, medium-sized toes (I' and I") lying close together
and parallel to or slightly convex towards each other. This condition
indicates that the 2 toes, together, are to be regarded as the
equivalent of the normal single toe occupying the same position. The
2 toes are, I conjecture, derived from the single toe by splitting.
The first series of changes consists of the increase in length of the
lateral element (I") and a corresponding decrease of the median
element (I'). In the last term of the series there are only 4 toes
on the foot, but the inner toe is not like the normal inner toe of
poultry, but is a much elongated I".
In the Silkie, also, the series begins with 2 small, closely-applied
toes (I' and I"). But when there are only 2 toes the lateral one
is usually much the larger. Typically this lateral toe is, as stated,
split, so that the nail is double, and the degree of splitting is
variable, in extreme cases involving half or more than half of the
toe. A second series of changes consists of the gradual reduction of
toe I' (often concomitantly with an increase in I") which may
end in its entire disappearance and thus reduce the number of toes to
5, but these are not equivalent to the 5 toes of the Houdans, since
the extra Houdan toes are I', I", and those of the reduced Silkie
are I"_a_ and I"_b_. Finally, in Silkies, the inner toe (I')
may split (more or less completely), and thus the 7-toed condition
arises. Moreover, in Houdans I have on one or two occasions found
the lateral element (I") bifid distally, resembling perfectly the
typical condition found in the Silkies.
A simple nomenclature is suggested for these various types of
extra-toes. The simple double-toed condition, as found commonly
in Houdans, may be called the _duplex_ type (D). The loss of
I' gives the _reduced duplex_ (D'). The case of split I", as
commonly seen in the Silkie, is the _triplex_ type (T); with the
loss of I' this becomes the _reduced triplex_ (T', not duplex!).
The 7-toed condition of Silkies may be called the _quadruplex type_
(Q); the combination split I' and single I" gives the _reduced
quadruplex_ (Q').[3]
[3] _E. g._, Pen 813, 935 ♀, embryo from egg of May 13.
The reduction that leads to the loss of I' consists of a loss
of phalanges, as Bateson (1904) has already pointed out. It seems
probable that the reduction affects first the proximal phalanges,
since the distal nail-bearing phalanx is the last to disappear.
B. RESULTS OF HYBRIDIZATION.
First let us consider the result of mating extra-toed individuals
belonging to "pure" extra-toed races. A typical Houdan cock (D
type), of the well-known Petersen strain, was mated with 3 hens bred
by me, but derived, several generations before, from the same strain.
With the first hen he got 29 chicks, all with the extra-toe except
one (3.3 per cent) that had 4 toes on both feet and two that had 4
toes on one foot and 5 on the other, _i. e._, one foot simplex and
one duplex. With the second he got 12 chicks, of which one had 4-5
(D) toes. The third, in 26 young, gave one with 4 toes on each
foot. Thus, in 67 chicks altogether there were 2, or 3 per cent,
with the normal number of toes on both feet (4-4). Unfortunately
these birds did not survive, so it is not known whether they would
have thrown as large a proportion of extra-toed offspring as
5-toed Houdans. Bateson's Dorkings gave about 4 per cent of 4-toed
offspring. Of the 83 offspring of 6-toed Silkies, 3, or 3.6 per
cent, had 4 toes on each foot. Even in pure-bred polydactyl races,
consequently, the character "extra-toe" does not uniformly appear in
the offspring.
Let us consider next what happens when a polydactyl individual is
crossed with a normal individual. Table 10 gives the results of
all matings of this sort and its most obvious result is that the
polydactyl condition reappears in every family, but not, as in
typically Mendelian cases, in _all_ of the offspring; at least this
is true of the Houdan crosses. In the Silkie crosses the 6 offspring
given as having the single thumb may possibly have been of the type
D', as that type was not in mind at the time of making the record
and was not always distinguished from type S. It is also clear
that the offspring of Silkie crosses are more apt to be polydactyl
than those of Houdan crosses. For 27 per cent of the latter are
non-polydactyl, while, taking the table as it stands, at most only
about 4 per cent and (as just stated) probably none of the Silkie
offspring were of the typical single-thumbed type. Also the average
degree of polydactylism is much greater in the Silkie than in the
Houdan crosses. This excess is in part due to the different method
of counting toes in the Silkie and the Houdan hybrids; for whereas
in the latter the visible toes are counted as equivalent units,
in the former in the case of each reduced type one unit more is
assigned than appears. The actual number of toes occurring in the
Silkie hybrids was also calculated, and it was found that this still
averaged higher than that of the Houdans (9.45 as opposed to 9.26).
TABLE 10.--_Frequency of the various types of toes in the first
hybrid generation between a normal and an extra-toed parent._
+-------------------------------------------------------------------------------------------------------------+
| A. HOUDAN CROSSES. |
+-----+--------------------------------------+---------------------------------+------------------------------+
| Pen | Mother. | Father. | Offspring. |
| No. +-----------+----------------+---------+---------------------------------+---------------------+--------+
| | No. | Race involved. | No. of | No. | Race involved. | No. of | Types of toes. |Average.|
| | | | toes. | | | toes. | 4-4 | 4-5 | 5-5 | |
+-----+-----------+----------------+---------+-------+----------------+--------+-------+------+------+--------+
| | {8 or 11 | Houdan. | 5-5 | } | | | { 0 | 1 | 8 | 9.9 |
| 504 | { 8 | Do. | 5-5 | } 13 | Wh. Leghorn. | 4-4 | { 1 | 3 | 8 | 9.6 |
| | { 11 | Do. | 5-5 | } | | | { 2 | 2 | 7 | 9.5 |
| | | | | | | | | | | |
| 525 | 8 or 11 | Do. | 5-5 | 27 | Minorca. | 4-4 | 8 | 3 | 13 | 9.2 |
| | | | | | | | | | | |
| 727 | { "Y" | Dk. Brahma. | 4-4 | } 831 | Houdan. | 5-5 | { 3 | 2 | 5 | 9.2 |
| | { 121 | Do. | 4-4 | } | | | { 13 | 9 | 18 | 9.1 |
| | | | | | | | | | | |
| 504 | 10-12 | Wh. Leghorn. | 4-4 | 9 | Do. | 5-5 | 3 | 2 | 0 | 8.4 |
| +-------+------+------+--------+
| Total (110) | 30 | 21 | 59 | 9.26 |
| Percentages | 27.3| 19.1| 53.6| |
+------------------------------------------------------------------------------+-------+------+------+--------+
| B. SILKIE CROSSES. |
+----+--------------------+-------------------------+---------------------------------------------------------+
| Pen| Mother. | Father. | Offspring. |
| No.+-----+-------+------+-----+------------+------+-------------------------------------------------+-------+
| | No. | Race. |No. of| No. | Race. |No. of| Types of toes.[A] | Aver- |
| | | | toes.| | | toes+---+----+---+-----+----+---+----+-----+----+-----+ age. |
| | | | | | | |ss.|sd'.|sd.|d'd'.|d'd.|dd.|st'.|d't'.|dt'.|t't'.| |
+----+-----+-------+------+-----+------------+------+---+----+---+-----+----+---+----+-----+----+-----+-------+
| 851| 1002|Cochin.| 4-4 | 7526|Silkie. | 6-6 |.. | .. | 1 | .. | 1 | 2 | .. | .. | 2 | 3 | 10.78 |
| 851| 3410| Do. | 4-4 | 7526| Do. | 6-6 |1? | .. |.. | .. | 2 | 7 | .. | .. | 1 | 3 | 10.43 |
| 815| 131| Do. | 4-4 | 774| Do. | 6-6 |.. | .. |.. | 1 | .. | 8 | .. | 1 | 1 | 1 | 10.33 |
| 851| 2073| Do. | 4-4 | 7526| Do. | 6-6 |.. | .. |.. | .. | .. | 7 | 1 | .. | .. | 1 | 10.33 |
| 734| 841| Do. | 4-4 | 774| Do. | 6-6 |.. | .. |.. | .. | .. | 3 | .. | .. | 1 | .. | 10.25 |
| 851| 838| Do. | 4-4 | 7526| Do. | 6-6 |.. | .. | 1 | 1 | .. |11 | .. | .. | .. | 3 | 10.25 |
| 851| 2299| Do. | 4-4 | 7526| Do. | 6-6 |.. | .. | 1?| 1 | .. | 4 | .. | .. | .. | 1 | 10.14 |
| 851| 5567| Do. | 4-4 | 7526| Do. | 6-6 |.. | .. |.. | .. | 1 |10 | 1 | .. | 1 | .. | 10.08 |
| 734| 840| Do. | 4-4 | 7526| Do. | 6-6 |.. | .. |.. | 1 | .. | 7 | .. | .. | .. | .. | 10.00 |
| 734| 1002| Do. | 4-4 | 774| Do. | 6-6 |.. | .. |.. | .. | 2 | 8 | .. | .. | .. | .. | 10.00 |
| 851| 840| Do. | 4-4 | 7526| Do. | 6-6 |.. | .. |.. | .. | .. | 4 | .. | .. | .. | .. | 10.00 |
| 851| 841| Do. | 4-4 | 7526| Do. | 6-6 |.. | .. |.. | .. | 1 | 1 | .. | .. | .. | .. | 10.00 |
| 744| 777|Silkie.|[B]5-6| 1176|Wh. Leghorn.| 4-4 |.. | .. |.. | .. | .. | 6 | .. | .. | .. | .. | 10.00 |
| 744| 496| Do. | 6-6 | 1176| Do. | 4-4 |1? | .. |.. | .. | .. |12 | .. | .. | 1 | .. | 9.93 |
| 851| 6956|Cochin.| 4-4 | 7526|Silkie. | 6-6 |4? | 1 |.. | 2 | .. | 3 | .. | .. | .. | .. | 9.50 |
| +---+----+---+-----+----+---+----+-----+----+-----+-------+
| Total (138) | 6 | 1 | 3 | 6 | 7 |93 | 2 | 1 | 7 | 12 | 10.13 |
+---------------------------------------------------+---+----+---+-----+----+---+----+-----+----+-----+-------+
[A] Of the reduced triplex type (t').
[B] s, means type of single thumb; d, duplex type; d', reduced duplex; t', reduced triplex.
In hybrids of both classes the greatest number of toes occurring on
one foot never exceeds the greatest number possessed by its parents;
indeed, the most polydactyl hybrids of the F1 generation of Silkies
never have as many as 6 toes on one foot. This result is not to be
explained as due to a regression towards the 4-4-toed condition, but
rather as due to the intermediate condition of the heterozygote. For
80 per cent of the hybrids show either the typical or the reduced
D type on one or both feet, although neither parent exhibits these
types.
We have next to consider the results of mating together the F1
hybrids. Table 11 gives the results of all matings of this sort.
TABLE 11.--_Frequency of the various types of toes in the second
hybrid generation between normal and extra-toed races. Lettering
as in table 10._
+------------------------------------------------------------------------------------------------------------------------+
| A. HOUDAN CROSSES (F1 × F1). |
+------+-----+------------------------------+------------------------------+---------------------------------------------+
| | | Mother. | Father. | Offspring. |
| | +-----+------------------+-----+-----+------------------+-----+----------------------------------+----------+
|Serial| Pen | | | | | | | | Average |
| No. | No. | | | No. | | | No. | Types of toes. | num. of |
| | | No. | Races involved. | of | No. | Races involved. | of +------+------+------+------+------+ toes per |
| | | | |toes.| | |toes.| 4-4 | 4-5 | 5-5 | 4-6 | 5-6 | bird. |
+------+-----+-----+------------------+-----+-----+------------------+-----+------+------+------+------+------+----------+
| 1 | 631 | 429 |Houd. × Wh. Legh. | 5-5 | 83 |Wh. Legh. × Houd. | 4-4 | 14[A]| 7 | 28 | 1 | .. | 9.3 |
| 2 | 728 | 174 | Do. | 5-5 | 258 | Do. | 5-5 | 11 | 1 | 20 | .. | .. | 9.3 |
| 3 | 631 | 448 | Do. | 5-5 | 409 | Do. | 4-4 | 13 | 4 | 18 | .. | .. | 9.1 |
| 4 | 637 | 529 |Houd. × Min. | 5-5 | 570 |Houd. × Min. | 4-4 | 4 | .. | 5 | .. | .. | 9.1 |
| 5 | 631 | 430 |Houd. × Wh. Legh. | 4-4 | 83 |Wh. Legh. × Houd. | 4-4 | 20 | 1 | 21 | .. | .. | 9.0 |
| 6 | 631 | 504 |Wh. Legh. × Houd. | 5-5 | 83 | Do. | 4-4 | 27 | 3 | 23 | .. | .. | 8.9 |
| 7 | 631 | 174 |Houd. × Wh. Legh. | 5-5 | 83 | Do. | 4-4 | 14 | 9 | 11 | .. | 1 | 8.9 |
| 8 | 519 | 85 | Do. | 4-5 | 83 | Do. | 4-4 | 9 | 2 | 4 | .. | .. | 8.7 |
| 9 | 637 | 569 |Houd. × Min. | 5-5 | 570 |Houd. × Min. | 4-4 | 14 | 1 | 4 | .. | 1 | 8.7 |
| 10 | 637 | 797 | Do. | 5-5 | 570 | Do. | 4-4 | 2 | .. | 1 | .. | .. | 8.7 |
| 11 | 631 | 86 |Houd. × Wh. Legh. | 4-4 | 83 |Houd. × Wh. Legh. | 4-4 | 11 | 1 | 6 | .. | .. | 8.7 |
| 12 | 637 | 685 |Houd. × Min. | 4-4 | 570 |Houd. × Min. | 4-4 | 5 | 1 | 2 | .. | .. | 8.6 |
| 13 | 631 | 84 |Houd. × Wh. Legh. | 4-4 | 83 |Houd. × Wh. Legh. | 4-4 | 17 | 13 | 4 | .. | .. | 8.6 |
| 14 | 519 | 84 | Do. | 4-4 | 83 | Do. | 4-4 | 7 | 1 | 2 | .. | .. | 8.5 |
| 15 | 519 | 86 |Wh. Legh. × Houd. | 4-4 | 83 |Wh. Legh. × Houd. | 4-4 | 12 | 2 | 2 | .. | .. | 8.4 |
| | | | +------+------+------+------+------+----------+
| | | | Totals (380) |180 | 46 | 151 | 1 | 2 | 8.92 |
| | | | Percentages | 47.4 | 12.1| 39.7| 0.3 | 0.5 | |
+------+-----+-----+-------------------------------------------------------+------+------+------+------+------+----------+
| B. SILKIE CROSSES (F1 × F1). |
+------+---+------------------------+------------------------+-----------------------------------------------------------+
| | | Mother. | Father. | Offspring (F2). |
| | +----+-------------+-----+----+-------------+-----+-----------------------------------------------------------+
|Serial|Pen| | | No. | | | No. | Types of toes. |
| No. |No.| No.| Races. | of | No.| Races. | of +-----------------------------------------------------------+
| | | | |toes.| | |toes.| ss | sd |d'd'| d'd| dd | st |d't'| dt'| dt |t't'| t't| tt |
+------+---+----+-------------+-----+----+-------------+-----+----+----+----+----+----+----+----+----+----+----+----+----+
| 16 |753|2071|Min. × Silk. | 4-4 |2573|Min. × Silk. | 4-5 | 7 | .. | .. | 1 | 19 | .. | 1 | .. | 3 | .. | 1 | .. |
| 17 |753|1966| Do. | 4-4 |2573| Do. | 4-5 | 12 | 2 | .. | .. | 15 | 1 | .. | .. | 2 | .. | .. | 4 |
| 18 |753|2575| Do. | 4-5 |2573| Do. | 4-5 | 18 | .. | 1 | .. | 16 | .. | .. | 1 | .. | .. | .. | 1 |
| 19 |709|3827|Silk. × Span.| 4-4 |1578|Silk. × Span.| 6-5 | 3 | .. | .. | .. | 2 | .. | .. | .. | .. | .. | .. | .. |
| 20 |709|1963| Do. | 4-4 |1578| Do. | 6-5 | 12 | 5 | .. | 1 | 15 | 1 | .. | .. | 1 | .. | .. | 1 |
| 21 |821|7413|Silk. × Coch.| 5-5 |6095|Silk. × Coch.| 5-5 | 1 | .. | .. | 1 | 7 | .. | .. | .. | 2 | .. | .. | .. |
| 22 |821|7423| Do. | 5-5 |6095| Do. | 5-5 | 3 | .. | .. | .. | 7 | .. | .. | .. | .. | 1 | .. | 1 |
| 23 |821|7428| Do. | 5-5 |6095| Do. | 5-5 | 5 | .. | 1 | 4 | 13 | .. | .. | 2 | .. | .. | .. | 1 |
| 24 |821|7408| Do. | 5-5 |6095| Do. | 5-5 | 3 | 1 | .. | .. | 8 | .. | .. | .. | 1 | 1 | .. | .. |
| | | | +----+----+----+----+----+----+----+----+----+----+----+----+
| | | | Total (208) | 64 | 8 | 2 | 7 |102 | 2 | 1 | 3 | 8 | 2 | 1 | 8 |
+------+---+----+--------------------------------------------+----+----+----+----+----+----+----+----+----+----+----+----+
[A]: Includes 1 case of 3-4 toes.
Comparing tables 10 and 11, it is at once clear that in the second
hybrid generation the proportion of extra-toed offspring has
decreased. This accords with expectation, if extra-toe is dominant,
for then only 75 per cent would be of the dominant type in F2, while
100 per cent would be of that type in F1.
Table 12 will enable us to analyze the difference of the proportions
in tables 10 and 11.
TABLE 12.--_Percentages of the various types of toes in F1 and F2
of the polydactyl hybrids compared._
+------+---------------+------------------+------------------+
| | _a_. | _b_. | _c_. |
| |Houdan hybrids.|Silkie hybrids (as|Silkie hybrids (as|
|No. of| | observed). | interpreted).[A]|
|toes. +---------------+------------------+------------------+
| | F1. | F2. | F1. | F2. | F1. | F2. |
+------+-------+-------+-------+----------+-------+----------+
| 4-4 | 27.3 | 47.4 | 9.4 | 31.7 | 4.3 | 30.8 |
| 4-5 | 19.1 | 12.1 | 9.4 | 7.7 | 2.9 | 3.8 |
| 4-6 | .... | .3 | .... | 1.0 | 1.5 | 1.0 |
| 5-5 | 53.6 | 39.7 | 81.2 | 51.4 | 76.8 | 53.4 |
| 5-6 | .... | .5 | .... | 4.3 | 5.8 | 5.8 |
| 6-6 | .... | .... | .... | 3.9 | 8.7 | 5.3 |
+------+-------+-------+-------+----------+-------+----------+
[A] Reduced duplex and triplex toes classified as typical duplex
and triplex.
These tables yield several points of interest. First, although the
proportions of normal and extra toe in table 12, _a_ and _c_, are
not Mendelian, yet the average _increase_, from F1 to F2 in the
proportion of the recessive (4-toed) type is almost exactly what is
called for by Mendel's law. That law calls for an increase of 25
per cent. The actual average increase is 23.3 per cent (20.1 and
26.5 in the two cases). It seems fair to conclude, consequently,
that Mendel's law does hold here, and that the 4-toed individuals
of F1 are heterozygotes with imperfect dominance. The feet of most
of the 4-toed Silkies of this generation belong, indeed, to the
reduced 5-toed type (table 10, B), and the reduced condition is
_prima facie_ evidence of heterozygotism. In F1 Silkies of the
first hybrid generation, 20 per cent of the feet exhibit "reduced"
types of toes, but in F2 only 5 per cent; and this might have been
anticipated, since in F2 heterozygotes are relatively only half as
numerous as in F1. Again, in F2 we see reappearing the high ancestral
toe-numbers (practically lost in the heterozygotes of F1, table 12,
_b_). These I interpret as extracted dominants. 6-toed extracts are
more numerous among the Silkie than the Houdan hybrids, because the
Silkie ancestors were 6-toed and the Houdan ancestors only 5-toed.
However, only a small proportion of the extracted Silkie dominants
have as many toes as the original Silkie ancestors, and this
indicates a permanent regression (through the contaminating influence
of hybridization?) toward the normal condition of toes. It will be
observed that, although 6 toes are not found in the Silkie hybrids
of F1, many of these heterozygotes are of the reduced triplex type.
Classifying them as virtually 6-toed, we find (table 12, _c_) 14.5
per cent of the 6-toed type in the F1 generation.
Among the extracted dominants of F2 are a few showing more toes than
appeared in the ancestors (table 12, _a_; there was also one 7-toed
F2 Silkie hybrid, not recorded in the table). It is this sort of
an advance in F2 that permits the breeder to make a forward step.
Theoretically, the appearance of this more aberrant class is probably
due to the greater numbers of progeny than of ancestors, since the
extracted dominants of F2 are seven times as numerous as their
extra-toed grandparents. Here, as elsewhere, the absolute range of
variability depends upon the number of individuals observed.
TABLE 13.--_Distribution of toe-numbers in the offspring of
DR × R matings._
+--------------------------------------------------------------------------------------------------------+
| A. HOUDAN CROSSES |
+------+---------------------------------+------------------------------+--------------------------------+
| | Mother. | Father. | Offspring. |
| +----+----+-----------------+-----+----+-------------------+-----+-----+-----+-----+-----+--------+
|Serial|No. | | | No | | | No. | | | | |Average |
| No. |of | No.| Races involved. | of | No.| Races involved. | of | 4-4 | 4-5 | 5-5 | 4-6 |num. of |
| |pen.| | |toes.| | |toes.|toes.|toes.|toes.|toes.|toes per|
| | | | | | | | | | | | | bird. |
+------+----+----+-----------------+-----+----+-------------------+-----+-----+-----+-----+-----+--------+
| 1 |519A| 87|Houd. × Wh. Legh.| 4-5 | 71| Wh. Legh. | 4-4 | 17 | 2 | 6 | .. | 8.6 |
| 2 |671 | 742|Min. × Dk. Brah. | 4-4 | 352| Houd. × Dk. Brah. | 4-4 | 8 | 2 | 2 | .. | 8.5 |
| | | | +-----+-----+-----+-----+--------+
| | | | Totals (37) | 25 | 4 | 8 | .. | 8.54 |
+------+----+----+------------------------------------------------------+-----+-----+-----+-----+--------+
| B. SILKIE CROSSES. |
+------+----+----+-----------------+-----+----+-------------------+-----+-----+-----+-----+-----+--------+
| 3 |706 | 10|Wh. Legh. | 4-4 |1965| Silkie × Spanish | 5-5 | 4 | .. | 4 | .. | 9.00 |
| 4 |766 |3814| Do. | 4-4 | 834| Blk. Game × Silkie| 5-5 | 10 | 4 | 8 | 1 | 9.00 |
| 5 |766 | 10| Do. | 4-4 | 834| Do. | 5-5 | 7 | .. | 5 | .. | 8.83 |
| 6 |607 | 203|Frizzle × Silkie | 5-5 | 15| Frizzle | 4-4 | 15 | 2 | 9 | .. | 8.77 |
| 7 |766 |3815|Wh. Legh. | 4-4 | 834| Blk. Game × Silkie| 5-5 | 11 | .. | 7 | .. | 8.77 |
| 8 |706 |3815| Do. | 4-4 |1965| Silkie × Spanish | 5-5 | 6 | .. | 3 | .. | 8.67 |
| 9 |706 | 71| Do. | 4-4 |3823| Do. | 5-5 | 18 | 1 | 8 | .. | 8.63 |
| 10 |766 |3832|Buff Legh. | 4-4 | 834| Blk. Game × Silkie| 5-5 | 7 | .. | 2 | .. | 8.44 |
| 11 |706 |3833| Do. | 4-4 |1965| Silkie × Spanish | 5-5 | 3 | 1 | | .. | 8.25 |
| 12 |607 | 230|Frizzle × Silkie | 4-4 | 15| Frizzle | 4-4 | 23 | 2 | 2 | .. | 8.22 |
| 13 |706 | 71|Wh. Legh. | 4-4 |1965| Silkie × Spanish | 5-5 | 5 | .. | .. | .. | 8.00 |
| 14 |706 |3814| Do. | 4-4 |1965| Do. | 5-5 | 6 | .. | .. | .. | 8.00 |
| 15 |706 |3832|Buff Legh. | 4-4 |1965| Do. | 5-5 | 5 | .. | .. | .. | 8.00 |
| | | | +-----+-----+-----+-----+--------+
| | | | Totals (179) | 120 | 10 | 48 | 1 | 8.60 |
+------+----+----+------------------------------------------------------+-----+-----+-----+-----+--------+
TABLE 14.--_Distribution of toe-numbers in the offspring of DR × D
matings._
+-------------------------------------------------------------------------------------------------------+
| A. HOUDAN CROSSES. |
+------+---+---------------------------+-------------------------+--------------------------------------+
| | | Mother. | Father. | Offspring. |
| | +---+-----------------+-----+----+---------------+----+-----+-----+-----+-----+-----+--------+
|Serial|Pen|No.| Races involved. | No. | No.|Races involved.| No.| 4-4 |4-5 | 5-5 | 5-6 | 6-6 |Average |
| No. |No.| | | of | | | of |toes.|toes.|toes.|toes.|toes.|num. of |
| | | | |toes.| | |toes| | | | | |toes per|
| | | | | | | | | | | | | | bird. |
+------+---+---+-----------------+-----+----+---------------+----+-----+-----+-----+-----+-----+--------+ bird.
| 1 |803|529|Houdan × Min. | 5-5 |7522|Houdan | 5-5| 1 | 4 | 13 | .. | .. | 9.67 |
+------+---+---+-----------------+-----+----+---------------+----+-----+-----+-----+-----+-----+--------+
| B. SILKIE CROSSES |
+------+---+---+-----------------+-----+----+---------------+----+-----+-----+-----+-----+-----+--------+
| 2 |606|182|Frizzle × Silkie.| 4-4 |775 |Silkie. | 6-6| .. | 3 | 10 | 3 | 5 | 10.48 |
| 3 |606|182| Do. | 4-4 | 21A| Do. | 6-6| .. | .. | 5 | .. | 1 | 10.33 |
| 4 |606|182| Do. | 4-4 |551 | Do. | 5-6| .. | .. | 5 | .. | .. | 10.00 |
| | | | +-----+-----+-----+-----+-----+--------+
| | | | Totals (32) | .. | .. | 20 | 3 | 6 | 10.36 |
+------+---+---+-------------------------------------------------+-----+-----+-----+-----+-----+--------+
TABLE 15.--_Percentages of the various types of toes in F1, F2,
DR × R and DR × D matings of the polydactyl crosses compared._
+--------+-----------------------------------+-----------------------------------+-----------------------------------+
| | | | _c._ Silkie crosses (reduced |
| | _a._ Houdan crosses. | _b._ Silkie crosses. | forms of toe classified as |
| No. of | | | typical). |
| toes. +--------+--------+--------+--------+--------+--------+--------+--------+--------+--------+--------+--------+
| | Mating | Mating | Mating | Mating | Mating | Mating | Mating | Mating | Mating | Mating | Mating | Mating |
| | F1. | F2. | DR × R | DR × D | F1. | F2. | DR × R | DR × D | F1. | F2. | DR × R | DR × D |
+--------+--------+--------+--------+--------+--------+--------+--------+--------+--------+--------+--------+--------+
| |_P. ct._|_P. ct._|_P. ct._|_P. ct._|_P. ct._|_P. ct._|_P. ct._|_P. ct._|_P. ct._|_P. ct._|_P. ct._|_P. ct._|
| 4-4 | 27.3 | 47.4 | 67.6 | 5.6 | 9.4 | 31.7 | 67.0 | .. | 4.3 | 30.8 | 66.7 | .. |
| 4-5 | 19.1 | 12.1 | 10.8 | 22.2 | 9.4 | 7.7 | 5.6 | 9.4 | 2.9 | 3.8 | 3.1 | 9.4 |
| 5-5 | 53.6 | 39.7 | 21.6 | 72.2 | 81.2 | 51.4 | 26.8 | 62.5 | 76.8 | 53.4 | 24.6 | 62.5 |
| 4-6 | .. | .3 | .. | .. | .. | 1.0 | .6 | .. | .. | 1.0 | 1.9 | .. |
| 5-6 | .. | .5 | .. | .. | .. | 4.3 | .. | 9.4 | 5.8 | 5.8 | 1.5 | 9.4 |
| 6-6 | .. | .. | .. | .. | .. | 3.9 | .. | .. | 8.7 | 5.3 | 1.2 | 18.7 |
| 6-7 | .. | .. | .. | .. | .. | .. | .. | .. | .. | .. | .. | .. |
+--------+--------+--------+--------+--------+--------+--------+--------+--------+--------+--------+--------+--------+
As we have seen, failure of dominance is much more complete in
some of the individuals of F2, namely, those with 4 toes, than
others. There is a variation in "potency." Is the degree of potency
inherited? Do the 4-toed heterozygotes produce a larger proportion of
imperfect dominants in F2 than the 5-toed heterozygotes? The answer
to this question should be given by the correlation between total
number of toes in the two parents and average number of toes in their
offspring, as given in table 11. In the case of the Houdan crosses
there is a strong positive correlation, measured by 0.683±0.092;
but the correlation is insignificant in the Silkie crosses
(-0.085±0.032). This lack of correlation in the Silkie hybrids is
perhaps due to the heavy regression in toe-number characteristic
of the second hybrid generation. In general, there seems to be an
inheritance of potency.
It now remains to test our conclusions by reference to the mating
of the heterozygote with the dominant and with the recessive types,
respectively. An examination of tables 13 to 15, particularly the
last, reveals several points of interest. Mendelian expectation
in the DR × R cross is 50 per cent of the recessive (4-4) type.
Actually, in the two crosses, A and B, 68 per cent and 67 per
cent, respectively, were obtained. But recalling that of these
amounts one-half of 27.3, or 13.71, and one-half of 9.4, or
4.7, are respectively due to failure to develop the extra-toe
in heterozygotes, there remain 54 per cent and 62 per cent,
respectively, of 4-toed offspring, which doubtless represent the
extracted RR type and approach the expected proportions.
Mendelian expectation in the DR × D cross (table 15) is 50 per
cent heterozygotes and 50 per cent extracted dominants. Of the
heterozygotes some 14 per cent may be expected to show 4-4 toes; that
the percentage is much less than that is doubtless due to the small
numbers involved. What is striking is the reappearance, in the second
generation, of large proportions of the extreme dominant type. These
results thus confirm those of the F2 generation.
Since extra-toe frequently fails to dominate, there should be certain
4-toed heterozygotes which throw extra-toe offspring, and such are
found. In table 16 are given six matings of 4-toed DR's. One sees
that they produce some 5-toed offspring. On the other hand, extracted
4-toed recessives are obtained, as table 17 shows.
Finally, we must consider whether, among the polydactyl birds of
one class, _e. g._, Houdans or Silkies, there is any difference
in the "centgener power" of parents corresponding to the degree
of development of their extra toes. This inquiry is suggested by
Castle's study (1906, p. 20) of polydactyl guinea-pigs. He finds that
when the extra toes of the mothers are graded into the 5 classes,
good (G), fair (F), poor (P), normal though of abnormal ancestry (N),
and normal of normal ancestry (N'), it follows: "first, that the
proportion of polydactylous young produced by a male decreases in
the successive classes from G to N'; and, secondly, that the degree
of development of the toes produced on those polydactylous young
diminishes in the same order." It is possible to test this conclusion
in poultry because, inside of any one type of extra-toe, _e. g._, the
triplex type, variation appears in the absolute size of the toes and
in the degree of their separateness. Our questions, then, are: (1)
does the _proportion of polydactyl young_ produced by a pair of birds
of any type diminish with the degree of development of toes inside
of that type, and (2) does the _degree of development_ of the toes
produced on the polydactylous offspring diminish in the same order?
TABLE 16.--_Distribution of toe-numbers in the offspring of
4-toed heterozygotes._
+-----+-------------------------+-----------------------+-----------------+--------+
| | Mother. | Father. | Offspring. | |
| Pen +----+-------------+------+----+-----------+------+-----+-----+-----+ Nature |
| No. | No.| Races. |No. of| No.| Races. |No. of| 4-4 | 4-5 | 5-5 | of |
| | | | toes.| | | toes.|toes.|toes.|toes.| mating.|
+-----+----+-------------+------+----+-----------+------------+-----+-----+--------+
| 637 | 685|Houd. × Min. | 4-4 | 570|Houd.×Min. | 4-4 | 5 | 1 | 2 |DR × DR |
| 729 | 913|Houd. × Min. | 4-4 | 936|Houd.×Legh.| 4-4 | 38 | 13 | 19 |DR × DR |
| 729 |2269| Do. | 4-4 | 936| Do. | 4-4 | 15 | 5 | 10 |DR × DR |
| 729 |2324| Do. | 4-4 | 936| Do. | 4-4 | 30 | 5 | 3 |DR × R |
| 642 | 750|Min. × Polish| 4-4 | 647| Do. | 4-4 | 10 | .. | 3 | R × DR |
| 671 | 742|Min. × Brah. | 4-4 | 352|Houd.×Brah.| 4-4 | 8 | 2 | 2 | R × DR |
+-----+----+-------------+------+----+-----------+------+-----+-----+-----+--------+
TABLE 17.--_Distribution of toe-numbers in the offspring of
extracted 4-toed parents._
+----+------------------------+--------------------------+-----------------+-------+
| | Mother. | Father. | Offspring. | |
| +----+-------------+-----+---+----------------+-----+-----+-----+-----+Nature |
|Pen | | | No. | | | No. | | | | of |
|No. | No.| Races. | of |No.| Races. | of | 4-4 |4-5 | 5-5 |mating.|
| | | |toes.| | |toes.|toes.|toes.|toes.| |
+----+----+-------------+-----+---+----------------+-----+-----+-----+-----+-------+
| {|2011|Polish × Min.| 4-4 |444|F2 Houd. × Legh.| 4-4 | 10 | .. | .. | R × R |
| {|2614| Do. | 4-4 |444| Do. | 4-4 | 6 | .. | .. | R × R |
|762{|2333| Do. | 4-4 |444| Do. | 4-4 | 16 | .. | .. | R × R |
| {|2618| Do. | 4-4 |444| Do. | 4-4 | 2 | .. | .. | R × R |
| {|3776| Do. | 4-4 |444| Do. | 4-4 | 2 | .. | .. | R × R |
+----+----+-------------+-----+---+----------------+-----+-----+-----+-----+-------+
Two sets of data are available for answering these questions. The
most direct set includes the data derived from crossing "pure-bred"
polydactyl birds and the other includes the data derived from using
hybrids between normal-toed and polydactyl ancestors. The latter data
have the advantage that the parents offer a greater variability;
but they have the disadvantage that the germinal condition of those
parents is incompletely known.
The pure races may be considered first. Eight matings of Houdans,
each parent with 5 toes, gave 122 offspring, of which 116 had 5-5
toes, 3 had 4-5 toes, and 3 had 4-4 toes. The variability of the toes
is not great in the parent Houdans. But, arranging them in the order
of development of the toes, the most developed first, the series of
table 18 results.
TABLE 18.
+------+---------+-------+--------------------------+
| | | | Offspring. |
|Serial| Pen No. |No. of +-----+-----+-----+--------+
| No. | |mother.| 4-4 | 4-5 | 5-5 |Average.|
| | | |toes.|toes.|toes.| |
+------+---------+-------+-----+-----+-----+--------+
| 1 | 727 803 | 2457 | 1 | 2 | 34 | 9.89 |
| 2 | 727 803 | 3105 | 1 | 0 | 45 | 9.95 |
| 3 | 803 | 2579 | .. | 1 | 12 | 9.92 |
| 4 | 727 | 3106 | .. | .. | 4 | 10.00 |
| 5 | 727 | 2494 | 1 | 0 | 5 | 9.67 |
| 6 | 727 | 2459 | .. | .. | 16 | 10.00 |
+------+---------+-------+-----+-----+-----+--------+
No direct relation here appears between development of the extra toe
in the parents and the average number of toes in the offspring.
Of the Silkies, 3 hens were used in 5 matings. The same 6-toed cock
(No. 774) was employed throughout (table 19).
TABLE 19.
+------+---------+----------+-------+---------------------------------+
| | | Mother. | | Offspring (No. of toes). |
|Serial| Pen No. +---+------+ f +---+---+---+---+---+---+---------+
| No. | |No.|No. of| |4-4|5-4|5-5|4-6|5-6|6-8| Average.|
| | | | toes.| | | | | | | | |
+------+---------+---+------+-------+---+---+---+---+---+---+---------+
| 1 | 734 815 |499| 6-6 |21{_a_ | 2 | 1 | 7 | 0 | 3 | 8 | 10.3 |
| | | | | {_b_ | 1 | 0 | 3 | 0 | 0 |17 | 11.4 |
| | | | | | | | | | | | |
| 2 | 734 815 |773| 6-5 |13{_a_ |.. |.. | 6 | 0 | 3 | 4 | 10.9 |
| | | | | {_b_ |.. |.. | 2 | 1 | 1 | 9 | 11.4 |
| | | | | | | | | | | | |
| 3 | 734 |500| 5-5 | 8{_a_ |.. | 2 | 4 | 0 | 2 |.. | 10.0 |
| | | | | {_b_ |.. |.. | 3 | 2 | 2 | 1 | 10.5 |
+------+---------+---+------+-------+---+---+---+---+---+---+---------+
In table 19 the series _a_ of observed average numbers of filial
toes (10.3, 10.9, 10.0) and the series _b_ obtained by assigning
the typical full number to all reduced types (11.4, 11.4, 10.5) are
decidedly irregular. There is, however, between the parental and the
filial series a correlation of +0.250±0.070. This indicates a slight
tendency for the number of toes in the progeny to vary with those of
the parentage.
The second set of data is derived from special matings made with
hybrids between Houdans and 4-toed races. On the one hand, in pens
728 and 813, cocks with well-developed toes of the duplex type were
mated with hens as nearly as possible of the same sort; while in
pens 765, 769, and 820 cocks with small, imperfectly separated toes
(probably of the duplex type[4]) were mated with hens as far as
possible of the same sort.
[4] I say probably of the duplex type because the cock
of pen 769 had a distally split toe on the right foot, reminding
somewhat of the _reduced_ triplex type. But as the left foot had a
typical duplex thumb, and the triplex is not common in Houdans, it
should probably be classed as duplex.
Tables 20, 21, and 22 give in detail and in summary the distribution
of types of polydactylism in the families from well-developed and in
those from poorly developed parents. They show a great difference
between the offspring of parents with good extra-toe (table 20) and
those with poor extra-toe (table 21). The former yield over 80 per
cent offspring with 5 toes or more on one or both feet, while the
latter yield about 57 per cent of such.
On the other hand, in the former families there are less than half
as many offspring with only 4 toes as in the latter. Classifying
"reduced" forms with their proper advanced type, we find highly
polydactyl parents yielding only 16 per cent non-polydactyl
offspring, while slightly polydactyl parents yield 43 per cent
non-polydactyl offspring. The percentage of polydactylous young
diminishes with the size and distinctness of the extra toes and the
grades of the polydactyl offspring are lower (absence in table 22, _b_,
of 6 toes). Both of Castle's conclusions seem to be confirmed.
TABLE 20.--_Distribution of toe-types in the offspring of "good"
extra-toed parents._
+------+---+---------------------------+---------------------------+---------+--------------------------------------+-------------------------------------------------------+
| | | Mother. | Father. | | Absolute numbers. | Theoretical classification. |
|Serial|Pen+----+----+-----------------+----+----+-----------------+ Mating. +-----+-----+-----+-----+-----+--------+----+----+-----+----+----+-----+----+---+-----+---+----+
| No. |No.| No.|Gen.| Races. | No.|Gen.| Races. | | 4-4 | 4-5 | 5-5 | 5-6 | 6-6 |Average.| ss.| sd.|d'd'.|d'd.| dd.|d't'.|dt'.|dt.|t't'.|tt.|q't.|
+------+---+----+----+-----------------+----+----+-----------------+---------+-----+-----+-----+-----+-----+--------+----+----+-----+----+----+-----+----+---+-----+---+----+
| 1 |728|2271| F2 |Wh. Legh. × Houd.| 258| F1 |Houd. × Wh. Legh.| DD × DR | 4 | 1 | 21 | .. | .. | 9.65 | 3 | .. | 1 | 1 | 21 | .. | .. | ..| .. | ..| .. |
| 2 |728| 912| F2 | Do. | 258| F1 | Do. | DR × DR | 5 | 3 | 21 | .. | .. | 9.55 | 5 | 3 | .. | .. | 20 | .. | 1 | ..| .. | ..| .. |
| 3 |728|2248| F2 | Do. | 258| F1 | Do. | DD × DR | 8 | 3 | 22 | .. | .. | 9.42 | 8 | 3 | .. | .. | 21 | .. | .. | ..| 1 | ..| .. |
| 4 |728|2272| F2 | Do. | 258| F1 | Do. | DR × DR | 17 | 4 | 34 | .. | .. | 9.31 | 17 | 1 | .. | 3 | 34 | .. | .. | ..| .. | ..| .. |
| 5 |728| 174| F1 | Do. | 258| F1 | Do. | DR × DR | 10 | 1 | 15 | .. | .. | 9.19 | 10 | 1 | .. | .. | 14 | .. | 1 | ..| .. | ..| .. |
| | | | | +-----+-----+-----+-----+-----+--------+----+----+-----+----+----+-----+----+---+-----+---+----+
| | | | | Totals (169) | 44 | 12 | 113 | .. | .. | 9.41 | 43 | 8 | 1 | 4 |110 | 0 | 2 | 0 | 1 | ..| .. |
| | | | | Percentages | 26.0| 7.1| 66.9| .. | .. | .. |25.4| 4.7| 0.6| 2.4|65.2| .. | 1.2| ..| 0.6 | ..| .. |
| | | | | +=====+=====+=====+=====+=====+========+====+====+=====+====+====+=====+====+===+=====+===+====+
| 6 |813|2271| F2 |Wh. Legh. × Houd.|3904| F3 |Houd. × Wh. Legh.| D × D | .. | 2 | 32 | .. | .. | 9.94 | .. | .. | .. | 2 | 32 | .. | .. | ..| .. | ..| .. |
| 7 |813|5113| F2 | Do. |3904| F3 | Do. | D × D | 2 | 1 | 32 | 1 | .. | 9.89 | .. | .. | 2 | 1 | 32 | .. | .. | 1 | .. | ..| .. |
| 8 |813| 377| F2 | Do. |3904| F3 | Do. | DR × D | 2 | 5 | 17 | .. | 1 | 9.68 | 2 | 2 | .. | 3 | 16 | .. | 1 | ..| .. | 1| .. |
| 9 |813|5122| F3 | Do. |3904| F3 | Do. | D × D | 1 | 3 | 7 | .. | .. | 9.55 | 1 | 3 | .. | .. | 7 | .. | .. | ..| .. | ..| .. |
| 10 |813| 935| F2 | Do. |3904| F3 | Do. | DR × D | 1 | 2 | 25 | 1 | 1 | 9.53 | 1 | 2 | .. | .. | 25 | .. | .. | 1 | .. | ..| 1 |
| 11 |813|2272| F2 | Do. |3904| F3 | Do. | DR × D | 5 | 2 | 18 | .. | .. | 9.52 | 4 | 1 | 1 | .. | 18 | 1 | .. | ..| .. | ..| .. |
| 12 |813| 912| F2 | Do. |3904| F3 | Do. | DR × D | 4 | 5 | 11 | .. | .. | 9.35 | 3 | 5 | 1 | .. | 11 | .. | .. | ..| .. | ..| .. |
| 13 |813|7320| F3 | Do. |3904| F3 | Do. | DR × D | 5 | 1 | 11 | .. | .. | 9.35 | 3 | 1 | 2 | .. | 11 | .. | .. | ..| .. | ..| .. |
| 14 |813|5142| F2 | Do. |3904| F3 | Do. | DR × D | 2 | 1 | 4 | .. | .. | 9.28 | 2 | .. | .. | 1 | 4 | .. | .. | ..| .. | ..| .. |
| | | | | +-----+-----+-----+-----+-----+--------+----+----+-----+----+----+-----+----+---+-----+---+----+
| | | | | Totals (205) | 22 | 22 | 157 | 2 | 2 | 9.70 | 16 | 14 | 6 | 7 |156 | 1 | 1 | 2| 0 | 1| 1 |
| | | | | Percentages | 10.7| 10.7| 76.5| 1.0 | 1.0 | .. | 7.8| 6.8| 2.9| 3.4|76.2| 0.5 | 0.5|1.0| .. |0.5| 0.5|
+------+---+----+----+-------------------------------------------------------+-----+-----+-----+-----+-----+--------+----+----+-----+----+----+-----+----+---+-----+---+----+
TABLE 21.--_Distribution of toe-types in the offspring of "poor" extra-toed parents._
+------+---+----------------------------+----------------------------+--------+-------------------------------+------------------------------------+
| | | Mother. | Father. | | Absolute numbers. | Theoretical classification. |
|Serial|Pen+----+-----+-----------------+----+-----+-----------------+ Mating.+-----+-----+-----+----+--------+----+----+-----+----+----+-----+----+
| No. |No.| No.| Gen.| Races. | No.| Gen.| Races. | | 4-4 | 4-5 | 5-5 |5-6 |Average.| ss.| sd.|d'd'.|d'd.| dd.|t't'.|dq'.|
| | | | | | | | | | | | | | | | | | | | | |
+------+---+----+-----+-----------------+----+-----+-----------------+--------+-----+-----+-----+----+--------+----+----+-----+----+----+-----+----+
| 1 |765| 984|F2 |Wh. Legh. × Houd.|1794|F2 |Wh. Legh. × Houd.| DR × DR| 9 | 5 | 11 | .. | 9.08 | 9 | 3 | .. | 2 | 10 | 1 | .. |
| 2 |765|1790|F2 | Do. |1794|F2 | Do. | DR × DR| 18 | 7 | 17 | .. | 8.98 | 18 | 6 | .. | 1 | 17 | .. | .. |
| | | | | +-----+-----+-----+----+--------+----+----+-----+----+----+-----+----+
| | | | | Totals (67) | 27 | 12 | 28 | .. | 9.02 | 27 | 9 | .. | 3 | 27 | 1 | .. |
| | | | | Percentages | 40.3| 17.9| 41.8| .. | .. |40.3|13.4| .. | 4.5|40.3| 1.5 | .. |
| | | | | +=====+=====+=====+====+========+====+====+====+====+====+=====+====+
| 3 |769| 492|F1 |Wh. Legh. × Houd.| 911|F2 |Wh. Legh. × Houd.| DR × DR| 13 | 1 | 14 | .. | 9.04 | 13 | 1 | .. | .. | 14 | .. | .. |
| 4 |769|4976|F2 | Do. | 911|F2 | Do. | DR × DR| 11 | 3 | 9 | .. | 8.91 | 11 | 3 | .. | .. | 8 | 1 | .. |
| 5 |769|2254|F2 | Do. | 911|F2 | Do. | DR × DR| 22 | 6 | 8 | .. | 8.61 | 22 | 4 | .. | 2 | 8 | .. | .. |
| 6 |769|1305|F2 | Do. | 911|F2 | Do. | DR × DR| 12 | 1 | 4 | .. | 8.53 | 12 | .. | .. | 1 | 4 | .. | .. |
| | | | | +-----+-----+-----+----+--------+----+----+-----+----+----+-----+----+
| | | | | Totals (104) | 58 | 11 | 35 | .. | 8.77 | 58 | 8 | .. | 3 | 34 | 1 | .. |
| | | | | Percentages | 55.8| 10.6| 33.7| .. | .. |55.8| 7.7| .. | 2.9|32.7| 1.0 | .. |
| | | | | +=====+=====+=====+====+========+====+====+=====+====+====+=====+====+
| 7 |820| 984|F2 |Wh. Legh. × Houd.|4731|F3 |Wh. Legh. × Houd.| D × DR| 2 | 3 | 27 | .. | 9.78 | 2 | 2 | .. | 1 | 27 | .. | .. |
| 8 |820|2255|F2 | Do. |4731|F3 | Do. | DR × DR| 6 | 1 | 10 | .. | 9.24 | 6 | .. | .. | 1 | 10 | .. | .. |
| 9 |820|6479|F3 | Do. |4731|F3 | Do. | DR × DR| 12 | 2 | 16 | .. | 9.13 | 10 | 1 | 2 | 1 | 15 | 1 | .. |
| 10 |820|2016|F1[A]| Do. |4731|F3 | Do. | DR × DR| 9 | 2 | 2 | .. | 8.45 | 9 | 2 | .. | ..| 2 | .. | .. |
| | | | | +-----+-----+-----+----+--------+----+----+-----+----+----+-----+----+
| | | | | Totals (92) | 29 | 8 | 55 | .. | 9.28 | 27 | 5 | 2 | 3 | 54 | 1 | .. |
| | | | | Percentages | 31.5| 8.7 | 59.8| .. | .. |29.3| 5.4| 2.2| 3.3|58.7| 1.1 | .. |
+------+---+----+-----+-------------------------------------------------------+-----+-----+-----+----+--------+----+----+-----+----+----+-----+----+
[A] No. 2016 has 4-4 toes and is a hybrid between a 5-toed White Leghorn × Houdan and a 4-toed Minorca × Polish.
But a more critical examination of the parentages of the 5 pens shows
that they are not comparable. In matings 6 to 14 of table 20 the
cock is almost certainly a dominant in respect to toes; whereas the
cocks in table 21 are probably heterozygous. The heterozygous state
determines two things: the imperfect nature of the extra-toe and a
relative deficiency in the offspring of the higher toe-numbers. In
our results we can not say that one of these things is the cause of
the other, as Castle does; they are, rather, in all probability, due
to a common cause. I think Castle's paper may justly be criticized
for not giving sufficient data concerning the ancestry of the
individual mothers used. Without such data the paper can not be said
satisfactorily to demonstrate his conclusion.
TABLE 22.--_Summary of observed toe-numbers in offspring,
percentages._
+-------------------------------------------++-----------------------------+
| _a._ Parents have "good" extra toes. || _b._ Parents have "poor" |
| || extra toes. |
+--------+------+------+------+------+------++--------+------+------+------+
| | 4-4 | 4-5 | 5-5 | 5-6 | 6-6 || | 4-4 | 4-5 | 5-5 |
| Pen No.| toes.| toes.| toes.| toes.| toes.|| Pen No.| toes.| toes.| toes.|
+--------+------+------+------+------+------++--------+------+------+------+
| 728 | 26.0 | 7.1 | 66.9 | .. | .. || 765 | 40.3 | 17.9 | 41.8 |
| 813 | 10.7 | 10.7 | 76.5 | 1.0 | 1.0 || 769 | 55.8 | 10.6 | 33.7 |
| | | | | | || 820 | 31.5 | 8.7 | 59.8 |
| +------+------+------+------+------++ +------+------+------+
|Average.| 17.7 | 9.1 | 72.2 | 0.5 | 0.5 ||Average.| 43.2 | 11.8 | 44.9 |
+--------+------+------+------+------+------++--------+------+------+------+
To summarize: "Potency," as measured by dominance of the extra-toed
condition, is inherited, in the Houdan crosses at least. There is
some evidence, derived from "pure-bred" Silkies, that differences in
the degree of development of the extra-toes are inherited. But the
average condition of the toes in the offspring of second or later
generation hybrids can not be used as evidence of inheritance of the
degree of parental development of the toes, since these are dependent
on the same basal cause, namely, the hidden gametic constitution
of the parents. Despite the obscuration of imperfect dominance,
polydactylism in poultry proves itself to be a unit-character that
segregates.
CHAPTER III.
SYNDACTYLISM.
A. STATEMENT OF PROBLEM.
In man, various mammals, and some birds two or more adjacent fingers
are sometimes intimately connected by an extension of the web that is
normally a mere rudiment at their base. Such a condition is known as
syndactylism. A good introductory account of syndactylism is given by
Bateson (1904, pp. 356-358). Taking a number of cases of syndactylism
together, he says: "A progressive series may be arranged showing
every condition, beginning from an imperfect webbing together of the
proximal phalanges to the state in which two digits are intimately
united even in their bones, and perhaps even to the condition in
which two digits are represented by a single digit." He also calls
attention to the fact that in the human hand "there is a considerable
preponderance of cases of union between the digits III and IV;" while
in the foot the united digits "are nearly always II and III." The
matter of syndactylism in birds has a peculiar interest because of
the fact that among wading and swimming birds syndactylism has become
a normal condition of the feet, and, moreover, just this feature
is one that has become classical in evolutionary history, because
Lamarck thought it well illustrated his idea of the origin of an
organ by effort and use.
Concerning the cause of syndactylism little can be said. Both in
mammals and birds the digits are indicated before they are freed from
lateral tissue connections. The linear development of the fingers
is in part accompanied by a cutting back of this primordial web, in
part by a growth beyond it. In syndactylism growth of the web keeps
pace with that of the fingers. From this point of view syndactylism
may be regarded as due to a peculiar excessive development of the
web.[5] In some human cases adhesions of the apex of the appendage to
the embryonic membranes has stimulated the growth of the interdigital
membrane, resulting in syndactylism. But it would be absurd to
attempt to explain syndactylism in general on this ground. The more
"normal" forms of syndactylism, as seen in poultry, still want for a
causal explanation.
[5] Lewis and Embleton (1908, p. 45) present strong
arguments against the theory that syndactylism is due to
arrested development.
Most of the cases of syndactylism whose inheritance is about to be
described arose in a single strain of fowl and can, indeed, be traced
back to a single bird. This ancestor is No. 121, a Dark Brahma hen
described in a previous report.[6] It was only in the search for the
origin of the exaggerated forms of syndactylism observed in some of
her descendants that an unusually great extension of the web in her
feet was noticed. The syndactyl condition of my birds did not, thus,
arise _de novo_, but had its origin antecedent to the beginning of
the breeding experiments. In addition to this main strain a slight
degree of syndactylism has appeared among some of my Cochin bantams.
[6] Davenport, 1906, page 34, Plate V.
TABLE 23.--_Ancestry of syndactyl fowl and the results of various
matings involving syndactylism._
[Abbreviations: _Ab_α, _Ab_β, etc., types of syndactylism (p. 32);
F, father; FF, father's father; FM, father's mother; M, mother;
MF, mother's father; MM, mother's mother; M × P, hybrid of Minorca
and Polish races; Synd., syndactyl (type unknown). f, foot. In
Nos. 24 to 42 two cocks (Nos. 242 and 3116, and 5399 and 4562,
respectively) were at different times used.]
+-----------+-----+-------------------------------------------------------------++-------------------------------------------------------------------------+
| | | First mating. || Second mating. |
| | +-------------------------------------+-----------------------++--------------------------------------------+---------------+------------+
| | | Ancestry. | Offspring. || Ancestry. | Offspring. | Average |
| Serial | Pen +-----+--------+-------+-----+--------+--------+--------------++-----+--------+-------+-----+--------+------+---------------+ per |
| No. | No. | M's | | | F's | | | Syndactyl. || M's | | | F's | | | Syndactyl. | cent |
| | | No. | MM. | MF. | No. | FM. | FF. +----+---+-----+| No. | MM. | MF. | No. | FM. | FF. +----+----+-----+ syndactyl. |
| | | | | | | | |2f. |1f.| 0f. || | | | | | | 2f.| 1f.| 0f. | |
+-----------+-----+-----+--------+-------+-----+--------+--------+----+---+-----++-----+--------+-------+-----+--------+------+----+----+-----+------------+
| | | | | | | | | | | || | | | | | | | | | |
| 1_a_, _b_ | 627 | 302 | [1]121 | [2]8A | 180 | [1]121 | [2]8A | 0 | 0 | 34 || 302 | [1]121 | [2]8A | 242 | [1]121 |[2]8A | 3 | 0 | 29 | 10.3 |
| 2_a_, _b_ | 627 | 280 | 121 | 8A | 180 | 121 | 8A | 0 | 0 | 23 || 280 | 121 | 8A | 242 | 121 | 8A | 2 | 0 | 21 | 9.5 |
| 3_a_, _b_ | 627 | 181 | 121 | 8A | 180 | 121 | 8A | 0 | 0 | 20 || 181 | 121 | 8A | 242 | 121 | 8A | 3 | 0 | 33 | 9.1 |
| 4_a_, _b_ | 627 | 354 | 121 | 8A | 180 | 121 | 8A | 0 | 0 | 24 || 354 | 121 | 8A | 242 | 121 | 8A | 1 | 0 | 37 | 2.6 |
| 5_a_, _b_ | 627 | 178 | 121 | 8A | 180 | 121 | 8A | 0 | 0 | 20 || 178 | 121 | 8A | 242 | 121 | 8A | 0 | 0 | 42 | .. |
| 6_a_, _b_ | 627 | 190 | 121 | 8A | 180 | 121 | 8A | 1 | 0 | 24 || 190 | 121 | 8A | 242 | 121 | 8A | 0 | 0 | 6 | .. |
| 7_a_, _b_ | ... | 353 | 121 | 8A | 180 | 121 | 8A | 0 | 0 | 13 || 353 | 121 | 8A | 242 | 121 | 8A | 0 | 0 | 22 | .. |
| 8_a_, _b_ | ... | 300 | 121 | 1A | 180 | 121 | 8A | 0 | 0 | 23 || 300 | 121 | 1A | 242 | 121 | 8A | 0 | 0 | 37 | .. |
| +----+---+-----+| +----+----+-----+ |
| Totals (182) | 1 | 0 | 181 || Totals (236) | 9 | 0 | 227 | |
| Percentages |0.55| 0 |99.45|| Percentages |3.81| 0 |96.19| |
+----------------------------------------------------------------+----+---+-----++--------------------------------------------+----+----+-----+------------+
+------+------+--------------------------+--------------------------+--------------------------------------------------------------------------------------+
| | | Mother. | Father. | Offspring. |
|Serial| Pen +------+-------+-----------+------+-------+-----------+----------------------+---------------------------------------------------------------+
| No. | No. | | Bred | | | Bred | | Syndactyl. | Classification. |
| | | No. |in pen | Toes. | No. |in pen | Toes. +----+---+-----+-------+------------+------------+-----------+------------+------------+
| | | | No. | | | No. | | 2f.|1f.| 0f. | P. ct.| _Aa_α. | _Ab_α. | _Ab_β. | _Ab_β´. | _Bb_α. |
+------+------+------+-------+-----------+------+-------+-----------+----+---+-----+-------+------------+------------+-----------+------------+------------+
| 9 | 747 | 2526 |[3]658 |Normal. | 1888 |[3]658 |Normal. | 9 | 0 | 9 | 50.0 | .. | 2 | 16 | .. | .. |
| 10 | 747 | 2831 | 658 | Do. | 1888 | 658 | Do. | 6 | 0 | 6 | 50.0 | .. | 7 | 5 | .. | .. |
| 11 | 747 | 2652 | 658 | Do. | 1888 | 658 | Do. | 3 | 0 | 25 | 10.7 | .. | 6 | .. | .. | .. |
| 12 | 747 | 3541 | 658 | Do. | 1888 | 658 | Do. | 4 | 0 | 41 | 8.9 | 1 | 4 | 3 | .. | .. |
| 13 | 747 | 1892 | 658 | Do. | 1888 | 658 | Do. | 4 | 0 | 47 | 7.8 | .. | .. | .. | .. | .. |
| 14 | 747 | 1872 | 658 | Do. | 1888 | 658 | Do. | 0 | 0 | 28 | 0.0 | .. | .. | .. | .. | .. |
| 15 | 747 | 1874 | 658 | Do. | 1888 | 658 | Do. | 0 | 0 | 28 | 0.0 | .. | .. | .. | .. | .. |
| | | | | | | | +----+---+-----+-------+ | | | | |
| | | | | | | | | 26 | 0 | 184 | 12.4 | | | | | |
| | | | | | | | +====+===+=====+=======+ | | | .. | .. |
| 16 | 703 | 2353 |D. Br. | Do. | 122 |D. Br. | Do. | 1 | 0 | 6 | 14.3 | .. | 2 | .. | .. | .. |
| 17 | 703 | 2030 |D. Br. | Do. | 122 |D. Br. | Do. | 2 | 1 | 12 | 20.0 | .. | 5 | .. | .. | .. |
| | | | | | | | +----+---+-----+-------+ | | | | |
| | | | | | | | | 3 | 1 | 18 | 18.2 | | | | | |
| | | | | | | | +====+===+=====+=======+ | | | .. | .. |
| 18 | 754 | 3126 |[4]627 |Normal. | 871 |[4]627 |Normal. | 12 | 1 | 30 | 30.2 | .. | 13 | 12 | .. | .. |
| 19 | 754 | 3175 | 627 | Do. | 871 | 627 | Do. | 3 | 0 | 8 | 27.3 | .. | 3 | 3 | .. | .. |
| 20 | 754 | 873 | 627 | Do. | 871 | 627 | Do. | [2]|(?)| (?)| (?) | .. | .. | 4 | .. | .. |
| 21 | 754 | 1052 | 627 | Do. | 871 | 627 | Do. | 0 | 0 | 17 | 0.0 | .. | .. | .. | .. | .. |
| 22 | 754 | 853 | 627 | Do. | 871 | 627 | Do. | 0 | 0 | 19 | 0.0 | .. | .. | .. | .. | .. |
| 23 | 754 | 862 | 627 | Do. | 871 | 627 | Do. | 0 | 0 | 27 | 0.0 | .. | .. | .. | .. | .. |
| | | | | | | | +----+---+-----+-------+ | | | | |
| | | | | | | | | 15 | 1 | 101 | 13.7 | | | | | |
| | | | | | | | +====+===+=====+=======+ | | | .. | .. |
| 24 | 767 | 2526 |[3]658 |Normal. | 3116 |D. Br. | Synd. | 5 | 0 | 22 | 18.5 | 1 | 1 | 6 | .. | 2 |
| 25 | 767 | 872 |[5]627 |_Ab_β | 242 |[5]513 |Normal. | 1 | 0 | 1 | 50.0 | .. | 1 | .. | 1 | .. |
| 25_a_| 767 | 872 | 627 |_Ab_β | 3116 |D. Br. | Synd. | 7 | 1 | 30 | 21.0 | 3 | 5 | 3 | .. | 4 |
| 26 | 767 | 2104 |[7]608 |Normal. | 3116 |D. Br. | Do. | 3 | 0 | 18 | 14.3 | .. | 2 | 2 | .. | 2 |
| 27 | 767 | 2831 |[3]658 | Do. | 3116 |D. Br. | Do. | 3 | 0 | 32 | 8.6 | .. | 6 | .. | .. | .. |
| 28 | 767 | 181 |[6]513 | Do. | 242 | 513 |Normal. | 1 | 0 | 22 | 4.4 | 2 | .. | .. | .. | .. |
| 28_a_| 767 | 181 | 513 | Do. | 3116 |D. Br. | Synd. | 1 | 1 | 60 | 3.2 | .. | 1 | 1 | .. | 1 |
| 29 | 767 | 190 |[5]520 | Do. | 242 | 513 |Normal. | 1 | 1 | 28 | 6.7 | 1 | .. | .. | .. | 2 |
| 29_a_| 767 | 190 | 520 | Do. | 3116 |D. Br. | Synd. | 4 | | 49 | 7.6 | .. | 3 | 4 | .. | 1 |
| +----+---+-----+-------+ | | | | |
| Syndactyl (242 ♂) | 3 | 1 | 51 | 7.3 | | | | | |
| Syndactyl (3116♂) | 23 | 2 | 211 | 9.4 | | | | | |
+-------------------------------------------------------------------+----+---+-----+-------+------------+------------+-----------+------------+------------+
[1] No. 121 is a Dark Brahma.
[2] No. 8A is a Tosa fowl (Game).
[3] (White Leghorn × Rose Comb Black Minorca) × Dark Brahma.
[4] Dark Brahma.
[5] See _supra_.
[6] 121 ♂ Dark Brahma × 8A Tosa.
[7] F2 (White Leghorn × Dark Brahma).
TABLE 23.--_Ancestry of syndactyl fowl and the results of various
matings involving syndactylism_--Continued.
+------+-----+-----------------------------------------------------+-------------------------------------------------------------------------------------+
| | | Mother. | Father. | Offspring. |
|Serial| Pen +------+-------+-----------+------+-------+-----------+---------------------+---------------------------------------------------------------+
| No. | No. | | Bred | | | Bred | | Syndactyl. | Classification. |
| | | No. |in pen | Toes. | No. |in pen | Toes. +----+---+-----+------+------------+------------+-----------+------------+------------+
| | | | No. | | | No. | |2f. |1f.| 0f. |P. ct.| _Aa_α. | _Ab_α. | _Ab_β. | _Ab_β´. | _Bb_α. |
+------+-----+------+-------+-----------+------+-------+-----------+----+---+-----+------+------------+------------+-----------+------------+------------+
| 30 | 801 | 4569 | 767 |_Ab_α | 5399 | 747 |_Ab_α | 2 | 0 | 0 |100.0 | 1 | 0 | 3 | 0 | 0 |
| 30_a_| 801 | 4569 | 767 |_Ab_α | 4562 | 767 |Normal. | 0 | 2 | 2 | 50.0 | .. | 1 | 1 | .. | .. |
| 31 | 801 | 6843 | 767 | Normal. | 4562 | 767 | Do. | 1 | 3 | 2 | 66.7 | .. | 2 | 2 | 1 | .. |
| 32 | 801 | 872 | 627 |_Ab_β | 5399 | 747 |_Ab_α | 12 | 4 | 11 | 59.3 | 3 | 9 | 11 | .. | 5 |
| 32_a_| 801 | 872 | 627 |_Ab_β | 4562 | 767 |Normal. | 7 | 1 | 12 | 40.0 | 2 | 8 | 4 | 1 | .. |
| 33 | 801 | 5515 | 767 |_Bb_α | 5399 | 747 |_Ab_α | 4 | 0 | 7 | 36.4 | .. | 2 | 6 | .. | .. |
| 33_a_| 801 | 5515 | 767 |_Bb_α | 4562 | 767 |Normal. | 1 | 2 | 5 | 37.5 | 2 | 1 | 1 | .. | .. |
| 34 | 801 | 7528 | 767 |_Ab_β | 5399 | 747 |_Ab_α | 1 | 0 | 0 |100.0 | .. | 2 | .. | .. | .. |
| 34_a_| 801 | 7528 | 767 |_Ab_β | 4562 | 767 |Normal. | 2 | 1 | 7 | 30.0 | .. | 1 | 4 | .. | .. |
| 35 | 801 | 6861 | 767 |Normal. | 4562 | 767 | Do. | 1 | 0 | 3 | 25.0 | .. | 2 | .. | .. | .. |
| 36 | 801 | 6869 | 767 | Do. | 5399 | 747 |_Ab_α | 0 | 1 | 3 | 25.0 | 1 | .. | .. | .. | .. |
| 36_a_| 801 | 6869 | 767 | Do. | 4562 | 767 |Normal. | 1 | 0 | 4 | 20.0 | .. | .. | 2 | .. | .. |
| 37 | 801 | 2831 | 658 | Do. | 5399 | 747 |_Ab_α | 3 | 1 | 18 | 18.2 | .. | 4 | .. | .. | 3 |
| 37_a_| 801 | 2831 | 658 | Do. | 4562 | 767 |Normal. | 2 | 1 | 11 | 21.4 | .. | 2 | .. | .. | 3 |
| 38 | 801 | 2526 | 658 | Do. | 5399 | 747 |_Ab_α | 0 | 0 | 5 | 0.0 | .. | .. | .. | .. | .. |
| 38_a_| 801 | 2526 | 658 | Do. | 4562 | 767 |Normal. | 1 | 0 | 2 | 33.3 | .. | 1 | 1 | .. | .. |
| 39 | 801 | 4570 | 767 | Do. | 5399 | 747 |_Ab_α | 0 | 1 | 5 | 16.7 | 1 | .. | .. | .. | .. |
| 39_a_| 801 | 4570 | 767 | Do. | 4562 | 767 |Normal. | 0 | 2 | 17 | 10.5 | 1 | 1 | .. | .. | .. |
| 40 | 801 | 1892 | 658 | Do. | 5399 | 747 |_Ab_α | 0 | 0 | 9 | 0.0 | .. | .. | .. | .. | .. |
| 40_a_| 801 | 1892 | 658 | Do. | 4562 | 767 |Normal. | 1 | 0 | 3 | 25.0 | .. | 2 | .. | .. | .. |
| 41 | 801 | 4263 | 767 | Do. | 5399 | 747 |_Ab_α | 0 | 1 | 4 | 20.0 | .. | 1 | .. | .. | .. |
| 41_a_| 801 | 4263 | 767 | Do. | 4562 | 767 |Normal. | 0 | 0 | 10 | 0.0 | .. | .. | .. | .. | .. |
| 42 | 801 | 6872 | 767 | Do. | 4562 | 767 | Do. | 0 | 0 | 6 | 0.0 | .. | .. | .. | .. | .. |
| +----+---+-----+------+ | | | | |
| Syndactyl (5399 ♂) | 22 | 8 | 62 | 32.6 | | | | | |
| Syndactyl (4562 ♂) | 17 |12 | 84 | 25.7 | | | | | |
| +====+===+=====+======+ | | | | |
| 43 | 776 | 2291 |Coch. |Normal. | 2732 |Coch. |Normal. | 2 | 0 | 6 | 25.0 | .. | 2 | .. | .. | 2 |
| 44 | 776 | 2574 |Coch. | Do. | 2732 |Coch. | Do. | .. | 1 | 9 | 10.0 | .. | 1 | .. | .. | .. |
| 45 | 776 | 2570 |Coch. | Do. | 2732 |Coch. | Do. | .. | 1 | 11 | 8.3 | .. | 1 | .. | .. | .. |
| 46 | 776 | 2297 |Coch. | Do. | 2732 |Coch. | Do. | .. | 1 | 12 | 7.7 | .. | .. | .. | .. | 1 |
| 47 | 776 | 2299 |Coch. | Do. | 2732 |Coch. | Do. | 1 | 0 | 16 | 5.9 | .. | 2 | .. | .. | .. |
| 48 | 776 | 2904 |Coch. | Do. | 2732 |Coch. | Do. | 0 | 0 | 6 | 0.0 | .. | .. | .. | .. | .. |
| 49 | 776 | 2937 |Coch. | Do. | 2732 |Coch. | Do. | 0 | 0 | 7 | 0.0 | .. | .. | .. | .. | .. |
| 50 | 776 | 2300 |Coch. | Do. | 2732 |Coch. | Do. | 0 | 0 | 15 | 0.0 | .. | .. | .. | .. | .. |
| 51 | 776 | 2736 |Coch. | Do. | 2732 |Coch. | Do. | 0 | 0 | 18 | 0.0 | .. | .. | .. | .. | .. |
| +----+---+-----+------+ | | | | |
| | 3 | 3 | 100 | 5.7 | | | | | |
| +====+===+=====+======+ | | | | |
| 52 | 816 | 121 |D. Br.|_Ab_α | 122 |D. Br. |Normal. | 3 | 1 | 10 | 28.6 | .. | 1 | .. | 2 | 4 |
| 52_a_| 816 | 121 |D. Br.|_Ab_α | 4912 |M × P | Do. | 0 | 0 | 13 | 0.0 | .. | .. | .. | .. | .. |
| 53 | 816 | 5835 |D. Br.|Normal. | 122 |D. Br. | Do. | 1 | 0 | 6 | 14.3 | .. | 2 | .. | .. | .. |
| 54 | 816 | 2353 |D. Br.| Do. | 122 |D. Br. | Do. | 0 | 0 | 7 | 0.0 | .. | .. | .. | .. | .. |
| 54_a_| 816 | 2353 |D. Br.| Do. | 4912 |M × P | Do. | 0 | 0 | 4 | 0.0 | .. | .. | .. | .. | .. |
| +----+---+-----+------+ | | | | |
| Syndactyl ( 122 ♂) | 4 | 1 | 23 | 17.9 | | | | | |
| Syndactyl (4912 ♂) | 0 | 0 | 17 | 0.0 | | | | | |
+------------------------------------------------------------------+----+---+-----+------+------------+------------+-----------+------------+------------+
The types of syndactylism which have appeared in my flock form a
rather extensive series. First, (_A_) the single web, which, in my
specimens, always occupies the interspace between digits III and
IV. This is the same interval which is most apt to show the web in
syndactylism of the human hand, and, it is suggestive to note, it
is this interval that is filled in those wading birds that have the
single web only between the toes (_e.g._, _Cursorius_, _Glareola_,
_Vanellus_, _Squatarola_, _Charadrius_, _Limosa_, _Machetes_,
_Himantopus_); second, there is (_B_) the double web, one-seventh
as common, which always occupies the interspaces between the digits
II-III and III-IV.
On another basis, the syndactyl feet may be classified as: (_a_) toes
adherent, web small in extent, and (_b_) toes distant, web broad.
I have found the narrow web only between digits III and IV. It is
one-eighth as common as the broad-webbed type. The broad, double web
approaches closely to the type found normally in swans, geese, and
ducks.
Finally, the syndactyl feet may be classified as: α,
straight-toed, or β, curve-toed. Class α is
to class β in frequency as 2:1. In the typical curve-toed
syndactyl foot the web between III and IV is complete to the nails of
each; in fact, in extreme cases the nails of the two toes are more or
less fused together. From the fused nails the middle toe, being the
longer, passes in a curve to the distal end of the metatarsus. The
D-shaped interspace between the curved III and straight IV toe is
filled with the web. In other cases the nails are merely approximated
and the middle toe is slightly curved. In three instances (4 per cent
of all) the outer toe (IV) is curved toward the (straight) median toe
(class β´).
As stated, the polydactyl offspring trace back their ancestry to
No. 121; her feet both show the double, broad, straight-toed type
(_Bb_α). We shall attempt in the following paragraphs to
trace the heredity of her type of polydactylism and of the others
that have subsequently arisen.
B. RESULTS OF HYBRIDIZATION.
In taking up the results of breeding experiments to test the method
of inheritance of syndactylism, it will be best first to give in
a table all pens in which the character showed itself, with the
frequency of the different types of foot in them (table 23).
The history of the syndactyl strain begins with No. 121 ♀ and
in the matings 1 to 8 are given the results of crossing together some
of her progeny derived from a normal-toed father. This father was
either No. 8A or 1A, both full-blooded Tosa (Japanese Game) fowl and
without suspicion in either soma or offspring of syndactyl taint.
There is no record of trace of syndactylism in the progeny of 121 ×
8A (or 1A); but a slightly developed condition of syndactylism may
very well have been overlooked by me in this F1 generation (as I had
never thought of such an abnormality), even as I at first overlooked
the syndactylism visible in No. 121. But when these F1 hybrids were
mated together (pen 627, serial Nos. 1 to 8) I got, in the different
families, from 10 per cent syndactyl offspring down to none at all.
At first sight the suggestion arises that, if inheritance is at
all Mendelian, the normal condition is dominant and that the
heterozygotes throw again, in pen 627, the syndactylous condition.
If this hypothesis were true it would follow that syndactyls bred
together should, sometimes at least, throw, even in large families,
100 per cent syndactyl offspring. But only 2 families, Nos. 30 and
34, have yielded 100 per cent syndactyls, and these contained 2 and
1 offspring, respectively; so they are not significant. On the other
hand, there are numerous matings of 2 extracted normal-toed parents
that have produced only normal-toed offspring (families Nos. 14, 15,
21, 22, 23, including 119 individuals). Consequently the conclusion
is favored that normal-foot is recessive and syndactyl-foot dominant,
and this shall be our working hypothesis.
On our hypothesis, No. 121 is probably a heterozygote. Mated with the
recessive normal, expectation is 50 per cent heterozygous, showing
syndactylism; the remainder normal-toed. But dominance is here, as
in polydactylism, very imperfect. For this reason and because it
was not looked for, no syndactylism was noted in the first hybrid
generation. The offspring prove to be of two sorts, however. No. 180
♂ is a pure recessive, and in 8 matings with as many different
sisters of his he got 184 normal-toed to 1 syndactyl. These same
sisters, mated to another brother, No. 242, in some cases gave 9 per
cent and 10 per cent syndactyl. No. 242 is, consequently, probably
a DR and, mated to DR sisters (which constitute according to
expectation about one-half of all) gives some DD's, part of which
constitute the 9 to 10 per cent of syndactyls. Of course, 25 per
cent DD is to be expected; the difference gives a measure in this
instance of the imperfection of dominance in the "extracted" as well
as "heterozygous" condition.
Matings 9 to 15 (pen 747) are instructive in comparison with the
foregoing case. Both parents are derived from pen 658, which
contained as breeders a heterozygous Dark Brahma male (No. 146) and
various females of non-booted races far removed from suspicion of
syndactylism; expectation being an equal number of DR and RR
offspring. In pen 747 No. 1888 ♂ acts like a DR, and so do the
hens in matings 9 to 13, while the hens in the other 2 matings are
doubtless RR's. The former give 17 per cent syndactyl offspring,
the latter none at all (in 56 individuals).
Matings 16 and 17 (pen 703) are between pure-bred Dark Brahmas
that are probably DR's. About 22 per cent of their offspring are
syndactyl--a rather higher proportion than we have found before.
Matings 18 to 19 are between progeny of pen 627. In mating 20 the
normals were not recorded. The cock in this pen, No. 871, is probably
heterozygous, as are also the first two hens, so that nearly 30
per cent of their progeny are syndactyl. From the other 3 hens no
syndactyl offspring were obtained. Evidently the two sets of hens
have a very different gametic constitution. The existence of two
sorts of families is one of the strong arguments for the segregation
of this character.
We next come to the pens (matings Nos. 24 to 42) which were
especially mated to study the inheritance of syndactylism. I had now,
for the first time, two parents with syndactylic feet.
On account of imperfection of dominance decision as to gametic
composition of any parent must largely rest on the make-up of the
progeny. Table 24 gives the most reasonable classification of the
parentages.
TABLE 24.
+-----------------------------------------------------------------------------------+
| DD × DD (SYNDACTYL × SYNDACTYL). |
+------+--------+-------+-----------+--------+-------+-----------+------------------+
| | | | | | | | Syndactyl. |
|Family|Mother's|Bred in| Toes. |Father's|Bred in| Toes. +---+---+---+------+
| No. | No. |pen No.| | No. |pen No.| |2t.|1t.|0t.|P. ct.|
+------+--------+-------+-----------+--------+-------+-----------+---+---+---+------+
| 30 | 4569 | 767 |_Ab_α | 5399 | 747 |_Ab_α | 2 | 0 | 0 | 100.0|
| 34 | 7528 | 767 |_Ab_β | 5399 | 747 |_Ab_α | 1 | 0 | 0 | 100.0|
| 32 | 872 | 627 |_Ab_β | 5399 | 747 |_Ab_α |12 | 4 |11 | 59.3 |
| 33 | 5515 | 767 |_Bb_α | 5399 | 747 |_Ab_α | 4 | 0 | 7 | 36.4 |
| +---+---+---+------+
| Totals |19 | 4 |18 | 74.2 |
+----------------------------------------------------------------+---+---+---+------+
| DD × DR. |
+------+--------+-------+-----------+--------+-------+-----------+---+---+---+------+
| 31 | 6843 | 767 |Normal. | 4562 | 767 |Normal. | 1 | 3 | 2 | 66.7 |
| 30_a_| 4569 | 767 |_Ab_α | 4562 | 767 | Do. | 0 | 2 | 2 | 50.0 |
| 33_a_| 5515 | 767 |_Bb_α | 4562 | 767 | Do. | 1 | 2 | 5 | 44.4 |
| 32_a_| 872 | 627 |_Ab_β | 4562 | 767 | Do. | 7 | 1 |12 | 42.9 |
| 34_a_| 7528 | 767 |_Ab_β | 4562 | 767 | Do. | 2 | 1 | 7 | 30.0 |
| 36 | 6869 | 767 |Normal. | 5399 | 747 |_Ab_α | 0 | 1 | 3 | 25.0 |
| 25_a_| 872 | 627 |_Ab_β | 3116 | D. Br.|Synd. | 7 | 1 |30 | 21.1 |
| 41 | 4263 | 767 |Normal. | 5399 | 747 |_Ab_α | 0 | 1 | 4 | 20.0 |
| 37 | 2831 | 658 | Do. | 5399 | 747 |_Ab_α | 3 | 1 |18 | 18.2 |
| 39 | 4570 | 658 | Do. | 5399 | 747 |_Ab_α | 0 | 1 | 5 | 16.7 |
| 40 | 1892 | 658 | Do. | 5399 | 747 |_Ab_α | 0 | 0 | 9 | 0.0 |
| +---+---+---+------+
| Totals |21 |14 |97 | 26.5 |
+----------------------------------------------------------------+---+---+---+------+
| DR × DR. |
+------+--------+-------+-----------+--------+-------+-----------+---+---+---+------+
| 38_a_| 2526 | 658 | Normal. | 4562 | 767 | Normal. | 1 | 0 | 2 | 33.3 |
| 35 | 6861 | 767 | Do. | 4562 | 767 | Do. | 1 | 0 | 3 | 25.0 |
| 40_a_| 1892 | 658 | Do. | 4562 | 767 | Do. | 1 | 0 | 3 | 25.0 |
| 37_a_| 2831 | 658 | Do. | 4562 | 767 | Do. | 2 | 1 |11 | 21.4 |
| 36_a_| 6869 | 767 | Do. | 4562 | 767 | Do. | 1 | 0 | 4 | 20.0 |
| 24 | 2526 | 658 | Do. | 3116 |D. Br. | Synd. | 5 | 0 |22 | 18.5 |
| 26 | 2104 | 608 | Do. | 3116 | Do. | Do. | 3 | 0 |18 | 14.3 |
| 39_a_| 4570 | 767 | Do. | 4562 | 767 | Do. | 0 | 2 |17 | 10.5 |
| 27 | 2831 | 658 | Do. | 3116 |D. Br. | Do. | 3 | 0 |32 | 8.6 |
| 29_a_| 190 | 520 | Do. | 3116 |D. Br. | Do. | 4 | 0 |49 | 7.6 |
| 29 | 767 | 190 | Do. | 242 | 513 | Do. | 1 | 1 |28 | 6.7 |
| 28_a_| 181 | 513 | Do. | 3116 | Do. | Do. | 1 | 1 |60 | 3.2 |
| +---+---+---+------+
| Totals |23 | 5 |249| 10.1 |
+----------------------------------------------------------------+---+---+---+------+
| RR × DR. |
+------+--------+-------+-----------+--------+-------+-----------+---+---+---+------+
| 42 | 6872 | 767 |Normal. | 4562 | 767 |Normal. | 0 | 0 | 6 | 0.0 |
| 41_a_| 4263 | 767 | Do. | 4562 | 767 | Do. | 0 | 0 |10 | 0.0 |
| +---+---+---+------+
| Totals | 0 | 0 |16 | 0.0 |
+----------------------------------------------------------------+---+---+---+------+
Summarizing the foregoing, and comparing the totals with Mendelian
expectation, we get the result shown in table 25.
A comparison of realization and expectation in table 25 shows that
the proportion of syndactyls is always less than expectation, not
only for dominants and heterozygotes together, but even for pure
dominants alone. The proportion of syndactyls obtained diminishes,
to be sure, in accordance with expectation (on the assumption that
they are pure dominants), but the numbers lag behind, in the higher
proportions 40 to 25 per cent. So we reach the conclusion that, as
in polydactylism, so in syndactylism dominance is very imperfect.
But there is this difference, that in syndactylism dominance is
so imperfect that the dominant condition rarely shows itself in
heterozygotes and even fails in many pure dominants. The striking
fact, the one that assures us the segregation is nevertheless
occurring in this case too, is that some families (whose two parents
are extracted recessives) throw 100 per cent recessives.
TABLE 25.
+-------------+-----+-----------------------------+--------------+
| | | Expectation. | Realization. |
| Nature of | +--------------+--------------+--------------+
| mating. | f | Dominants + | Pure | |
| | |heterozygotes.| dominants. | Syndactyls. |
+-------------+-----+--------------+--------------+--------------+
| | | _P. ct._ | _P. ct._ | _P. ct._ |
| DD × DD | 41 | 100.0 | 100.0 | 56.1 |
| DD × DR | 132 | 100.0 | 50.0 | 26.5 |
| DR × DR | 277 | 75.0 | 25.0 | 10.1 |
| RR × DR | 16 | 50.0 | 0.0 | 0.0 |
| RR × RR | 119 | 0.0 | 0.0 | 0.0 |
+-------------+-----+--------------+--------------+--------------+
These studies on syndactylism in poultry may be used for a critical
examination of the recent work of Lewis and Embleton (1908) on
syndactylism in man. The cases described by them follow the types I
have just described in poultry. Their fig. 18 corresponds to my types
_a_ and α; figs. 10 and 11 to my type β. The "crossbones"
referred to by the authors correspond to bones of the "curved toe."
The facts presented by the authors support the idea that syndactylism
is dominant rather than recessive, but they deny the application of
Mendelian principles to this case. Actually, the foot deformities
described by Lewis and Embleton are inherited much like syndactylism
in poultry. No extracted normal (recessive) extremity produces
the abnormal condition. Heterozygotes show much variation, from
very abnormal to slightly abnormal (possibly perfectly normal?)
appendages. Dominance is, indeed, much more potent than in poultry.
The authors' denial of the application of Mendelism to this case
seems to be based on an all too superficial consideration of the
hereditary behavior of the character and a tendency to "mass"
statistics--a procedure that tends to obscure the interpretation of
the data of heredity.
As to the inheritance of type, my statistics are not extensive enough
to give a final answer, but if all types be grouped into those with
straight and those with curved toes, then in crosses of straight-toed
syndactyl and normal 33 per cent of the offspring were of the curved
type, whereas in crosses of curved-toed syndactyls and normal 45 per
cent were of the curved type. These averages depend on 22 and 15
individuals, respectively. They lead us to look for an inheritance of
type when more extensive data shall be available.
Syndactylism is a typical sport, that is, a rather large mutation
having a teratological aspect. The question arises, Does it prove to
be prejudicial to the welfare of the species? The breeder who has
only a few individuals of a rare sport feels their loss more than
that of normals and the general impression left in his mind is that
the sport is less capable of maintaining itself than the normal form.
Assembling the data, consisting of about 40 individuals of each kind,
it appears that the death-rate is not very different in the two lots;
the slight excess of that of the syndactyls is sufficiently accounted
for by the circumstance that no _normals_ were reared during the
period of greatest mortality (the summer), but were destroyed or
given away as soon as hatched. It is probable, therefore, that
syndactylism, under the conditions of the poultry-yard, has little
life and death significance, but is one of those neutral characters
whose existence Darwin clearly recognized.
CHAPTER IV.
RUMPLESSNESS.
The tail of vertebrates is, historically, the post-anal part of the
trunk. Containing no longer any part of the alimentary canal, it has
lost much of its primitive importance, so that its disappearance in
any case is a matter of relatively little importance. Accordingly
we find groups of animals in which it is rudimentary or wholly
absent, such as many amphibia and the anthropoid apes and man. In
all recent birds the tail is a distinct but much reduced organ--the
uropygium--which contains several vertebræ in a degenerate condition.
The uropygium supports the tail feathers, which are of much use in
directing the bird in flight, but in ground birds, such as the grouse
and poultry, seem to function only for display in the male and, in
the female, to facilitate copulation.
Now, among various typically tailed vertebrates the tail is sometimes
absent. Tailless dogs, cats, sheep, and horses are known; on the
other hand, several cases of tails in man have been described
(Harrison, 1901). Thus the tail is a part of the body subject to
sporting; and it has also become the differential character for some
specific groups. In other words, it is an organ that has played
an important part in evolution and consequently its method of
inheritance is a matter of great interest.
The origin of the tailless poultry which I have bred has been
twofold. The most important strain is that referred to in an earlier
report[7] as Bantam Games. The second lot consists of rumpless fowl
that have arisen in my yards, spontaneously, from normal blood. Of
these more later.
[7] Davenport, 1906, pages 62 to 64, fig. 46.
The two rumpless Game cocks bore the numbers 117 and 116. Dr. A. G.
Phelps, of Glens Falls, New York, from whom the birds were purchased,
wrote that he had imported No. 117 from England, and No. 116 was its
son. The birds were very closely similar in all external features.
The matings made with No. 117 and their results are given in table 26.
TABLE 26.--_Progeny of tailless cock and tailed hens._
+------+------+--------+--------------------------+-----------------------------------+
| | | | Mother. | Offspring. |
| | | +----+---------------------+-------------------------+---------+
|Serial| Pen |Father's| | | Condition of uropygium. |Per cent |
| No. | No. | No. | No.| Races. |--------+------+---------|rumpless.|
| | | | | |Present.|Small.| Absent. | |
+------+------+--------+----+---------------------+--------+------+---------+---------+
| 1 |525 | 117 |114 | Nankin. | 3 | .. | 0 | 0 |
| 2 |526 | 117 | 20A| Frizzle. | 8 | .. | 0 | 0 |
| 3 |532 | 117 | .. |Bl. Coch. | 14 | .. | 0 | 0 |
| 4 |532_a_| 117 |127 |Wh. Legh. | 19 | .. | 0 | 0 |
| 4_a_|653 | 117 |508 |Bl. Coch. × Wh. Legh.| 8 | 3 | 0 | 0 |
| | | | | +--------+------+---------+---------+
| | | | | Totals | 52 | 3 | 0 | 0 |
+------+------+--------+----+---------------------+--------+------+---------+---------+
In 25 cases of the 52 an oil-gland was looked for and, in every case,
it was found to be missing.
Table 26, the conclusions from which were drawn in my 1906 report,
seemed to indicate the dominance of tail over its absence. On this
hypothesis I suspected that if No. 117 were bred to his (tailed)
offspring about 50 per cent of the progeny would be tailless, and if
the tailed hybrids of the F1 were bred together about 25 per cent of
their progeny should be tailless. The actual result of such matings
is shown in table 27.
TABLE 27.--_Heterozygotes mated with father._
+------+----+-------------------------------+-----------------------+
| | | Tailless cock × heterozygotes.| Offspring. |
| | +---------------+---------------+-----------------------+
|Serial| Pen| Father. | Mother. |Condition of uropygium.|
| No. | No.+-----+---------+-----+---------+--------+------+-------+
| | | No. | From | No. | From |Present.|Small.|Absent.|
| | | | pen No. | | pen No. | | | |
+------+----+-----+---------+-----+---------+--------+------+-------+
| 5 | 653| 117 |Original.| 577 | 532 | 6 | 1 | 0 |
| 6 | 653| 117 | Do. | 587 | 532 | 8 | 2 | 0 |
| 7 | 653| 117 | Do. | 635 | 532 | 7 | 0 | 0 |
| 8 | 653| 117 | Do. | 691 | 532 | 5 | 2 | 0 |
| 9 | 653| 117 | Do. | 652 | 532 | 15 | 0 | 0 |
| 10 | 653| 117 | Do. | 691 | 532 | 5 | 2 | 0 |
| 11 | 653| 117 | Do. | 705 | 532 | 9 | 2 | 0 |
| 12 | 653| 117 | Do. | 713 | 532 | 7 | 2 | 0 |
| 13 | 653| 117 | Do. | 760 | 532 | 13 | 2 | 0 |
| 14 | 653| 117 | Do. | 799 | 532 | 7 | 0 | 0 |
| | | +--------+------+-------+
| | | Total | 82 | 13 | 0 |
+------+----+-------------------------------+--------+------+-------+
TABLE 28.--_Heterozygotes mated inter se._
+------+----+----------+-----------+-----------------------------------------------+
| | | | | Condition of uropygium in offspring. |
| | | Father. | Mother. |-----------------------+-----------------------+
|Serial| Pen| | | Frequency. | Percentage. |
| No. | No.+----+-----+-----+-----+--------+------+-------+--------+------+-------+
| | | |From | |From | | | | | | |
| | | No.| pen | No. | pen |Present.|Small.|Absent.|Present.|Small.|Absent.|
| | | | No. | | No. | | | | | | |
+------+----+----+-----+-----+-----+--------+------+-------+--------+------+-------+
| 15 | 661| 466| 526 | 401A| 526 | 5 | 0 | 0 | 100 | 0 | 0 |
| 16 | 661| 466| 526 | 635 | 532 | 5 | 0 | 0 | 100 | 0 | 0 |
| 17 | 661| 466| 526 | 691 | 532 | 4 | 0 | 0 | 100 | 0 | 0 |
| 18 | 661| 466| 526 | 799 | 532 | 4 | 1 | 0 | 80 | 20 | 0 |
| 19 | 649| 516| 532A| 521 | 532A| 17 | 4 | 0 | 81 | 19 | 0 |
| 20 | 649| 516| 532A| 565 | 532A| 24 | 7 | 0 | 77 | 23 | 0 |
| 21 | 649| 516| 532A| 665 | 532A| 11 | 4 | 0 | 73 | 27 | 0 |
| 22 | 649| 516| 532A| 692 | 532A| 18 | 1 | 0 | 95 | 5 | 0 |
| 23 | 652| 343| 525 | 344 | 525 | 8 | 2 | 0 | 80 | 20 | 0 |
| 24 | 661| 428| 526 | 635 | 532 | 4 | 0 | 0 | 100 | 0 | 0 |
| 25 | 661| 428| 526 | 691 | 532 | 3 | 0 | 0 | 100 | 0 | 0 |
| 26 | 661| 428| 526 | 799 | 532 | 5 | 0 | 0 | 100 | 0 | 0 |
| +--------+------+-------+--------+------+-------+
| Total | 108 | 19 | 0 | 85 | 15 | 0 |
+----------------------------------+--------+------+-------+--------+------+-------+
The results given in tables 27 and 28 are remarkable. Neither in
the DR × R nor the DR × DR crosses did the tail fail to
develop. The tailless condition, that I had strongly suspected of
being recessive and expected in 25 per cent to 50 per cent of the
offspring, never once appeared. The only point of variation in the
uropygium of the chicks derived from the back cross or from F1's
bred _inter se_ was that in some the uropygium seemed distinctly
smaller than in the others. This small uropygium was as a matter
of fact recorded chiefly in chicks that failed to hatch, but it was
occasionally noticed in older birds, being then usually associated
with a slight convexity of the back. In some of the families the
uropygium is recorded as small in suspiciously close to 25 per cent
of the offspring. There is little doubt in my mind that this small
uropygium represents in some way the "absence" of tail that was
expected.
The next step was to cross the other rumpless bantam (No. 116),
to see if he behaved like his father. Accordingly, in pen 653, I
replaced the cock No. 117 by 116, the hens remaining the same, and
got the result shown in table 29.
TABLE 29.--_Heterozygotes mated with No. 116._
+------+------------+----------+----------------------------------------+
| | | | Condition of uropygium in offspring. |
|Serial| Father's | Mother's +----------+--------+---------+----------+
| No. | No. | No. | Present. | Small. | Absent. | Per cent |
| | | | | | | absent. |
+------+------------+----------+----------+--------+---------+----------+
| 27 | 116 | 508 | 5 | 2 | 10 | 59 |
| 28 | 116 | 577 | 3 | 0 | 3 | 50 |
| 29 | 116 | 587 | 3 | 1 | 4 | 50 |
| 30 | 116 | 652 | 4 | 0 | 2 | 33 |
| 31 | 116 | 705 | 3 | 1 | 5 | 56 |
| 32 | 116 | 713 | 1 | 0 | 2 | 67 |
| 33 | 116 | 760 | 4 | 0 | 2 | 33 |
| +----------+--------+---------+----------+
| Totals (55) | 23 | 4 | 28 | 51 |
+------------------------------+----------+--------+---------+----------+
Here we get a result almost exactly in accord with Mendelian
expectation. Having, now, obtained rumpless hens, it became possible
for the first time to test the inheritance of rumplessness in both
parents. The result is shown in the table 30.
TABLE 30.--_Rumpless fowl mated inter se._
+------+------+---------------+---------------+----------------------------+
| | | Father. | Mother. | Condition of tail in |
| | | | | offspring. |
| | +------+--------+------+--------+----------+--------+--------+
|Serial| Pen | | | | | | | |
| No. | No. | | From | | From | | | |
| | | No. | Serial | No. | Serial | Present. | Small. | Absent.|
| | | | No. | | No. | | | |
+------+------+------+--------+------+--------+----------+--------+--------+
| 34 | 742 | 2978 | 27 | 2601 | 29 | 0 | 0 | 4 |
| 35 | 854 | 2978 | 27 | 3430 | 27 | 0 | 0 | 9 |
| 36 | 742 | 2978 | 27 | 3430 | .. | [A]2 | 0 | 7 |
| 37 | 854 | 2978 | 27 | 2977 | 27 | [B]1 | 0 | 1 |
| +----------+--------+--------+
| Total | 3 | 0 | 21 |
+---------------------------------------------+----------+--------+--------+
[A] Both from chicks that died in shell.
[B] From a hatched chicken.
Table 30 is unfortunately small; one may say, fragmentary. Rumpless
hens are incapable of copulating unless the tail coverts are trimmed;
moreover my birds have been so much inbred that they are very weak;
finally, the chicks are so small that it is impracticable to rear
them in brooders and the eggs are particularly apt to be broken by
the brooding hens. However, it suffices to show that two tailless
fowl are able to throw some tailed offspring.
The second lot of rumpless fowl, namely, those that arose _de novo_
in my yards, must now be considered. In 1906, 2 birds hatched out
from ordinary tailed strains. As one was a cock and the other a
hen these were mated in 1907. The cock (No. 2464) came from No.
71♀ (a pure White Leghorn bred by myself from original White
Leghorn stock described in my 1906 report) and No. 235♂ (an F1
hybrid between one of these White Leghorns and my original Rose-comb
Black Minorca). The hen was No. 1636. Her mother (No. 618) was an F1
hybrid between a Minorca and Dark Brahma of series V, 1906 report,
and her father (No. 637) had the same origin. Thus the parents and
grandparents of both of these new rumpless birds were well known to
me and known to be fully tailed and to throw only tailed birds, with
the exception of these two birds.
The result of the mating of Nos. 2464 and 1636 in pen 736 was 25
chicks, of which 24 had tails and 1 (No. 5335) was without tail or
oil-gland. This, unfortunately, died early, so it was impossible to
breed it. In 1908, the hen No. 1636 having in the meantime died, I
mated No. 2464♂ to 6 of his (tailed) daughters. He was not well
and soon died, leaving no descendants by them, but 5 offspring by a
female cousin, all tailed. Then one of his sons (tailed) was mated
to its own sisters and produced 49 offspring, all tailed. Thus the
strain seems to have died out. The whole history is important both
because an apparently new mutation had taken place and because it
was, in a degree, "hereditary."
How, if at all, can this case and those of the bantams be brought
under known laws of inheritance? First of all, it must be confessed
that the provisional hypothesis, suggested in my earlier report, that
rumplessness is in my strain recessive has not been supported by the
newer facts. In the light of the principle of imperfect dominance
to which the facts of the last two chapters have led us, everything
receives a satisfactory explanation. The only conclusion that meets
all the facts is this: _The inhibitor of tail development--the
tailless factor--is dominant_; _its absence_--permitting a
continuation of the normal development of the tail region--_is
recessive_.
The application of this hypothesis to the various matings may now be
attempted. No. 117 is to be regarded as a heterozygote. The matings
with tailed birds is of the order DR × R, and expectation in the
typical case is 50 per cent DR (interrupted tail) and 50 per cent
RR (non-interrupted). But, owing to the relatively weak potency
of the interrupter derived from No. 117, growth of the tail is not
interrupted in the heterozygous offspring. These offspring are, by
hypothesis, so far as their gametes go, of two equally numerous
sorts, DR and RR. Mated to No. 117, two sorts of families are
to be expected, namely, the products of DR × RR (=50 per cent
DR, 50 per cent RR) and the products of DR × DR (=25 per
cent DD, 50 per cent DR, 25 per cent RR). The first lot of
families might be expected to resemble the preceding generation in
consisting entirely of tailed birds; the latter might be expected to
show in the 25 per cent extracted DD's evidence of the presence
of the undiluted interrupter. Actually in matings of the latter sort
(table 27) 3 families show no trace of the tail-interrupter, but in
7 there is evidence of a disturbance, as shown by the small size of
the uropygium and the bent back. In these families there are 13 cases
of small uropygium to 53 of large, being about 20 per cent of the
affected uropygium where 25 per cent was to be looked for--not a wide
departure, considering the liability of not recognizing the reduced
uropygium as such. This failure even of the extracted dominants
completely to stop the development of the tail gives a measure of the
weakness of the inhibitor in this case. Also, in table 28, matings
are varied. Some are probably matings of two heterozygotes, others of
two recessives, and others still of a recessive with a heterozygote.
On our hypothesis we should expect some of the families of the mated
hybrids to show evidence of the inhibiting factor and others to show
no such evidence. In those families in which small tail appears
it is found in about 19 per cent of the cases. On account of this
weakness of the inhibitor in the germ-plasm of No. 117 that inhibitor
is rarely fully activated. Only in one case out of the 250 or more
in which that germ-plasm is used is the development of the tail
completely stopped. In this case a hybrid cock derived from pen 526
(series 2, table 26) was crossed with various birds of tailed races
(probable RR's), and produced in addition to 20 tailed offspring 1
devoid of uropygium and oil-gland. In this case we may conceive that
an unusually potent condition of the inhibitor wholly stopped the
development of the tail.
The behavior of No. 116 is that of a pure dominant. Mated to
DR (and some RR?) females he produces pure dominants and
heterozygotes. His inhibiting factor is potent enough to be active in
the DD offspring at least; as a matter of fact 47 per cent of his
get have their tails inhibited. Even in the DR's the inhibitor may
sometimes work itself out. Thus No. 116 crossed on No. 508, without
tailless ancestry, had 56 per cent of the progeny without tail.
Since tailless birds may be either pure dominants or DR's, we may
expect families of two sorts when two such are bred together--those
containing only tailless offspring and those containing only 75 per
cent or less of such. Both sorts of families are to be expected in a
table with the composition of table 30, and both appear there.
The case of the rumpless fowl that arose _de novo_ will be explained,
then, as follows: Even in normal RR matings the inhibiting factor
may arise by mutation. But even when two of these inhibiting factors
are paired they show themselves so weak as not to appear in 25 per
cent, much less the typical 75 per cent of cases, but, as in our
case, merely 4 per cent. The strain takes on, indeed, the essential
features of the "eversporting varieties" of De Vries (1905). It seems
probable, therefore, that even in eversporting varieties inheritance
may be Mendelian, modified by variations in "potency" as shown by
irregularities in dominance.
CHAPTER V.
WINGLESSNESS.
The entire absence of appendages is a rare monstrosity, few cases
having been cited even for man. In my experience with poultry, out
of about 14,000 birds I have obtained one that had no wing on one
side of the body, but this unfortunately died before being bred from.
A second bird was given to me by a fancier. The bird was an Indian
Game, a vigorous cock, which was handicapped by his abnormality in
two ways. First, whenever he fell upon his side or back he was unable
to get upon his feet without aid. On several occasions he evidently
had spent hours upon the ground before he was discovered and picked
up. The wings are thus clearly most important to the fowl in enabling
it to regain its feet after having become prone. Secondly, he was
unable to tread a hen, since this act requires the use of wings as
balancers. He was, however, able to copulate with small birds without
leaving the ground. Thus in two respects his abnormality would have
proved fatal in nature. First, because of the personal risk, the
greater since a prone bird must fall an easy prey to predaceous
enemies; and secondly, because of the risk to his germ-plasm. Little
wonder, then, that this abnormality should not be known among wild
ground-birds.
Mated to 6 hens this wingless cock produced 130 chicks in 1907, of
which all had two wings. The following year he was mated to his
daughters, but died without leaving offspring. So I used a son of his
to mate with his own sisters and half-sisters. The progeny in this F2
generation consisted of 223 chicks, all of which had two wings. Thus,
no trace of winglessness appeared in any of the descendants of the
wingless cock.
The explanation of this case is not very certain, in view of the
limited data. It seems to resemble the behavior of No. 117, the
rumpless cock. And following the interpretation given in his case I
would conclude that winglessness is dominant to the normal condition,
that the original wingless cock was a heterozygote, and that the
dominance of winglessness was imperfect in the first generation. On
this hypothesis his son may well have been a pure recessive, and
then all of his descendants, in turn, would be either recessives
or heterozygotes (with imperfect dominance). It is, on the other
hand, possible that the wingless cock was a pure dominant, but that
the potency of the inhibitor was so slight as not to appear in the
heterozygotes or even in extracted dominants.
CHAPTER VI.
BOOTING.
The method of inheritance of the feathering on the feet of some
poultry has already been made the subject of much study. Hurst
(1905, p. 152) crossed booted and non-booted birds and bred the
hybrids together. He concluded that "the Mendelian principles are at
work in these aberrant phenomena, but are masked by something not
yet perceived." My own conclusion (1906, p. 72) was: "Booting is
dominant, but usually imperfectly so." A more extended study has been
desirable.
Booting is variable in amount. To indicate its degree I have had
recourse to an artificial scale. I recognize 11 grades, running from
0 to 10. The grade 0 implies no feathers whatsoever. Grade 10 implies
heavy booting extending over the front half of the shank. Grade 5
implies an extent of only half of the maximum, _i. e._, the outer
front quarter of the shank. Intermediate grades indicate intermediate
extension of the feathered area.
A. TYPES OF BOOTING.
The races of booted poultry used have been as follows: First, bantam
Cochins of two varieties; second, a bantam Dark Brahma; and third,
the Silkie. In my representatives of the first two groups, but
particularly in the Dark Brahma, the amount of booting is variable.
In one type the outer third of the shank in the newly hatched
chick is covered by strong, heavy, specialized feathers, directed
outward, while the middle and inner thirds are covered by smaller,
finer, imbricating feathers sparsely placed and resembling reduced
contour-feathers. In most individuals the transition from the one
kind to the other is gradual, while in others it is sharp, and in a
few the outer third only of the shank is feathered. In the Silkies,
which the standard poultry books describe as being more sparsely
feathered on the shank,[8] the outer zone of feathers is the only one
developed; and, occasionally, as table 31 shows, even these feathers
may be lacking. We have thus two types to distinguish--the extended
(Cochin, Brahma) type and the restricted type.
[8] Thus Wright (1902) says the shanks of the Silkies
(in England) are "slightly feathered," and Baldanus (1896) says
that (in Germany) they are feathered on the _outer half_.
B. NORMAL VARIABILITY.
To appreciate the results of hybridizing we must first examine the
variability of pure-blooded races. This is done in table 31.
TABLE 31.--_Distribution of boot-grades in the offspring of
Cochin, Dark Brahma, and Silkie parents._
+---------------------------------------------------------------------------------------------+
| A. OFFSPRING OF COCHIN PARENTS. |
+----+------------+------------+--------------------------------------------------------------+
| | Mother. | Father. | Grades of boot in offspring. |
|Pen |-----+------+------+-----+----+---+----+----+----+----+----+----+----+----+----+--------+
|No. | | Boot-| |Boot-| | | | | | | | | | | | |
| | No.| grade| No. |grade| 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 |Average.|
+----+-----+------+------+-----+----+---+----+----+----+----+----+----+----+----+----+--------+
|848 | 2297| 10 | 545 | 10 |.. | ..| .. | 1 | .. | .. | 1 | .. | .. | 1 | 18 | 9.43 |
|776 | 2574| 10 | 2732 | 8 |.. | ..| .. | .. | .. | .. | .. | .. | 3 | 2 | 6 | 9.27 |
|848 | 2300| 8 | 545 | 10 |.. | ..| .. | .. | .. | .. | .. | .. | 1 | 2 | 5 | 9.25 |
|776 | 2570| 6 | 2732 | 8 |.. | ..| .. | .. | .. | .. | 1 | 1 | .. | 11 | 1 | 8.71 |
|848 | 2075| 9 | 545 | 10 |.. | ..| .. | .. | .. | 1 | 1 | .. | .. | .. | 4 | 8.50 |
|776 | 2072| 6 | 2732 | 8 |.. | ..| .. | .. | .. | .. | 1 | .. | 4 | 2 | 2 | 8.44 |
|758 | 130| 6 | 545 | 10 |.. | ..| .. | .. | .. | 1 | 1 | 1 | 3 | 9 | .. | 8.20 |
|776 | 2073| 6 | 2732 | 8 |.. | ..| .. | .. | 1 | 2 | .. | 2 | 2 | 10 | 1 | 8.00 |
|776 | 2300| 6 | 2732 | 8 |.. | ..| .. | .. | 1 | .. | 1 | 3 | 6 | 5 | 2 | 8.00 |
|758 | 131| 10 | 545 | 10 |.. | ..| .. | .. | .. | 1 | .. | 4 | 6 | 1 | 1 | 7.96 |
|776 | 2297| 6 | 2732 | 8 |.. | ..| .. | 1 | .. | 1 | .. | 3 | 6 | 6 | 2 | 7.95 |
|776 | 1132| 3 | 2732 | 8 |.. | ..| .. | 1 | 1 | 1 | 1 | 3 | 6 | 8 | .. | 7.57 |
|776 | 2937| 7 | 2732 | 8 |.. | ..| .. | .. | .. | .. | 1 | 3 | 3 | 1 | .. | 7.50 |
|776 | 2299| 7 | 2732 | 8 |.. | ..| 1 | .. | .. | 1 | 1 | 4 | 7 | 3 | 1 | 7.44 |
| +----|---+----+----+----+----+----+----+----+----+----+--------+
| Totals (199) |.. | ..| 1 | 3 | 3 | 8 | 9 | 24 | 47 | 61 | 43 | 8.24 |
+------------------------------+----+---+----+----+----+----+----+----+----+----+----+--------+
| B. OFFSPRING OF DARK BRAHMA PARENTS. |
| [All individuals have sprung from No. 121 ♀ (boot of grade 9) and No. 122 ♂ |
| (boot of grade 6).] |
+---------------------------------------------------------------------------------------------+
|816 | 2030| 6 | 122 | 6 | .. | ..| .. | .. | .. | .. | .. | .. | .. | 1 | 3 | 9.8 |
|703 | 2030| 6 | 122 | 6 | .. | ..| .. | .. | .. | .. | 4 | 2 | 0 | 3 | 6 | 8.3 |
|816 | 121| 6 | 122 | 6 | .. | ..| .. | .. | .. | .1 | 3 | 1 | 2 | 4 | 5 | 8.3 |
|816 | 5979| 6 | 122 | 6 | .. | ..| .. | .. | .. | .. | 1 | 0 | 2 | .. | .. | 7.3 |
|816 | 2353| 5 | 122 | 6 | .. | ..| .. | .. |[A]1| 1 | 1 | 0 | 1 | 0 | 2 | 7.1 |
|816 | 5835| 5 | 122 | 6 | .. | ..| .. |[A]1| 0 | 1 | 2 | .. | .. | 1 | 3 | 6.5 |
|816 | 5840| 5 | 122 | 6 | .. | ..| .. |[A]1| .. | .. | 1 | .. | .. | .. | 1 | 6.3 |
|703 | 2353| 5 | 122 | 6 | .. | ..| .. | .. | 1 | 1 | 3 | .. | 1 | .. | .. | 5.8 |
| +----+---+----+----+----+----+----+----+----+----+----+--------+
| Totals (61) | .. | ..| .. | 2 | 2 | 4 | 15 | 3 | 6 | 9 | 20 | 7.62 |
+------------------------------+----+---+----+----+----+----+----+----+----+----+----+--------+
| C. OFFSPRING OF SILKIE PARENTS. |
+----+-----+------+------+-----+----+---+----+----+----+----+----+----+----+----+----+--------+
|734 | 468| 4 | 774 | 3 | .. | ..| 1 | 2 | .. | .. | 1 | 1 | .. | .. | .. | 4.20 |
|734 | 1002| 3 | 774 | 3 | .. | ..| 1 | 4 | .. | 1 | 3 | .. | .. | .. | .. | 4.11 |
|734 | 841| (?)| 774 | 3 | .. | ..| .. | .. | 2 | .. | .. | .. | .. | .. | .. | 4.00 |
|815 | 7434| 7 | 774 | 3 | .. | ..| .. | .. | 2 | .. | .. | .. | .. | .. | .. | 4.00 |
|734 | 773| 1 | 774 | 3 | .. | ..| .. | 2 | 2 | .. | .. | .. | .. | .. | .. | 3.50 |
|734 | 680| 1 | 774 | 3 | .. | ..| .. | 2 | .. | .. | .. | .. | .. | .. | .. | 3.00 |
|734 | 405A| 1 | 774 | 3 | .. | ..| 1 | 3 | 1 | .. | .. | .. | .. | .. | .. | 3.00 |
|815 | 499| 2 | 774 | 3 | 1 | 1| 3 | .. | .. | 2 | .. | 1 | .. | .. | .. | 3.00 |
|734 | 499| 2 | 774 | 3 | 1 | 1| 5 | 2 | 2 | 1 | .. | .. | .. | .. | .. | 2.50 |
|734 | 500| 1 | 774 | 3 | 2 | 1| 2 | 3 | .. | .. | .. | .. | .. | .. | .. | 1.75 |
|815 | 773| 1 | 774 | 3 | 4 | 1| 3 | .. | .. | .. | .. | .. | .. | .. | .. | 1.25 |
|815 | 500| 1 | 774 | 3 | 1 | 1| .. | .. | .. | .. | .. | .. | .. | .. | .. | 0.50 |
|815 | 496| 3 | 774 | 3 | 1 | ..| .. | .. | .. | .. | .. | .. | .. | .. | .. | 0.00 |
| +----+---+----+----+----+----+----+----+----+----+----+--------+
| Totals(68) | 10 | 5| 16 | 18 | 9 | 4 | 4 | 2 | .. | .. | .. | 2.72 |
+------------------------------+----+---+----+----+----+----+----+----+----+----+----+--------+
| SUMMARY. |
+------------------------------+--------------------------------------------------------------+
| | Grades of boot in offspring, reduced to percentages. |
| Races. +----+---+----+----+----+----+----+----+----+----+----+--------+
| | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 |Average.|
+------------------------------+----+---+----+----+----+----+----+----+----+----+----+--------+
|Cochins | .. | ..| 0.5| 1.5| 1.5| 4.0| 4.5|12.1|23.6|30.7|21.6| 8.24 |
|Dark Brahmas | .. | ..| .. | 3.3| 3.3| 6.6|24.6| 4.9| 9.8|14.8|32.8| 7.62 |
|Silkie |14.8|7.4|23.5|26.5|13.2| 5.9| 5.9| 2.9| .. | .. | .. | 2.72 |
+------------------------------+----+---+----+----+----+----+----+----+----+----+----+--------+
[A] Determination made on embryo chicks.
An inspection of table 31 shows that, in respect to booting, the
Cochins and Dark Brahmas are clearly closely related to each other.
Owing to smaller numbers and to other circumstances that will be
discussed later, the results are less regular in the Dark Brahma
offspring, but in both the range is from 2 or 3 upward to 10, with
a great preponderance in grades above 5. In the Silkies, on the
other hand, the greatest frequency is found in grades below 5. This
difference is correlated with a difference of the parents, for the
commonest grades of the parents of the Cochins are between 6 and 10,
of the Dark Brahmas between 5 and 9, and of the Silkies between 1 and
3. These results suggest that the Silkie is typically heterozygous in
boot, producing 25 per cent recessives (boot of grade 4-7) and 75 per
cent dominant (0, 1) and heterozygous (2, 3). We shall see that this
hypothesis receives support from all Silkie matings.
Inside of any part of this table it appears that, on the whole, as
the average grade of the boot in the progeny diminishes that of
the parentage diminishes, although the correlation is by no means
perfect. Thus the average of the parental grades in the first part of
table 31, A (which is arranged in descending order of the averages
of the offspring) is 8.5; in the lower half, 7.4. The average of
parental grades in the upper half of table 31, B is 6.4; in the lower
half 5.5. In table 31, C the grades are 2.9 and 2.3, respectively.
This correlation indicates, without exactly measuring, heredity in
grade of booting.
Table 32 shows the results of crosses between Cochins (high grade of
boot) and Silkies (low grade).
TABLE 32.--_Distribution of boot-grades between a high and low
grade of boot in parents._
+---------------------------------------------------------------------------------------------------+
| HIGH AND LOW GRADE OF BOOT IN PARENTS. |
+---+-----------------------------+-----------------------+-----------------------------------------+
| | Mother. | Father. | Grade of boot in offspring. |
|Pen|-----+----+-------------+----+-----+----+-------+----+--+--+--+--+--+--+--+--+--+--+--+--------+
|No.| | | | | | | | | | | | | | | | | | | | |
| | No. |Gen.| Races. |Gra.| No. |Gen.| Race. |Gra.| 0| 1| 2| 3| 4| 5| 6| 7| 8| 9|10|Average.|
+---+-----+----+-------------+----+-----+----+-------+----+--+--+--+--+--+--+--+--+--+--+--+--------+
|851| 5567| P | Bl. × Bf. C.| 9 | 7526| P | Silkie| 3 |..|..|..|..| 2|..|..|..| 3| 3| 5| 8.15 |
|851| 3410| P | Do. | 9 | 7526| P | Do. | 3 |..|..|..|..|..| 4| 3| 2| 1| 6| 1| 7.29 |
|851| 6956| P | Do. | 8 | 7526| P | Do. | 3 |..|..|..|..| 3| 3|..| 2| 2|..| 5| 7.13 |
|851| 2073| P | Do. | 7 | 7526| P | Do. | 3 |..| 1|..| 1| 1|..| 1| 1| 1| 3| 2| 6.91 |
|851| 2299| P | Do. | 7 | 7526| P | Do. | 3 |..|..|..|..| 2| 2| 1| 1|..|..| 3| 6.78 |
|851| 840| P | Bf. C. | 10 | 7526| P | Do. | 3 |..|..|..|..| 1|..| 1|..|..| 1|..| 6.33 |
|851| 1002| P | Do. | 8 | 7526| P | Do. | 3 |..|..|..| 3| 1| 2| 1| 2| 4| 1| 1| 6.27 |
|815| 131| P | Bk. C. | 10 | 774| P | Do. | 4 |..|..|..| 3| 1| 1| 2| 2| 1| 1| 2| 6.23 |
|851| 841| P | Bf. C. | 10 | 7526| P | Do. | 3 |..|..|..|..| 1|..| 1|..| 1|..|..| 6.00 |
|851| 838| P | Do. | 8 | 7526| P | Do. | 3 |..|..|..| 4| 2| 4| 3|..|..| 2| 2| 5.65 |
| |--+--+--+--+--+--+--+--+--+--+--+--------+
| Totals (116) | 0| 1| 0|11|14|16|13|10|13|17|21| 6.77 |
+---------------------------------------------------------+--+--+--+--+--+--+--+--+--+--+--+--------+
So far as the average grade of boot in offspring goes, this table
stands between that of the Cochins (table 31, A) and that of the
Silkies (table 31, C). But what is especially striking is the
apparent dimorphism revealed in the line of totals. There is one
(empirical) mode at 10, corresponding with that of the Cochins, and
a second clear mode at 5, corresponding to that of the Silkies. If
we assume the Cochin to be homozygous in boot (RR) and the Silkie
to be heterozygous in boot, then we can interpret the high mode as
extracted recessives, the median mode as heterozygotes.
C. RESULTS OF HYBRIDIZATION.
We have next to consider the nature of the inheritance when one
parent belongs to an unbooted race, the other to a booted one (table
33).
TABLE 33.--_Distribution of boot-grades in the F1 generation of booted × non-booted parents._
+------------------------------------------------------------------------------------------------------------------+
| A. COCHIN CROSSES. |
+---+----------------------------+----------------------------+----------------------------------------------------+
|Pen| Mother. | Father. | Grade of boot in offspring. |
|No.|----+----+-------------+----+-----+----+------------+----+---+---+---+---+---+---+---+---+---+---+---+--------+
| | No.|Gen.| Races. |Gra.| No. |Gen.| Races. |Gra.| 0| 1| 2| 3| 4| 5| 6| 7| 8| 9| 10|Average.|
+---+----+----+-------------+----+-----+----+------------+----+---+---+---+---+---+---+---+---+---+---+---+--------+
|773|1334| P | W. Legh. | 0 | 836| P | Bl. Coch. | 10 | ..| ..| ..| 3| 1| 1| 1| 1| ..| 2| ..| 5.44 |
|773| 193| P | Do. | 0 | 836| P | Do. | 10 | ..| 1| 2| 6| 8| 7| 4| 2| ..| ..| ..| 4.27 |
|773|1366| P | Do. | 0 | 836| P | Do. | 10 | ..| ..| ..| 2| 5| 2| 1| ..| ..| ..| ..| 4.20 |
|773| 127| P | Do. | 0 | 836| P | Do. | 10 | ..| ..| 3| 10| 9| 12| 4| ..| ..| ..| ..| 4.11 |
|773| 692| P | W. Legh. (R)| 0 | 836| P | Do. | 10 | ..| ..| ..| 10| 3| 2| ..| ..| ..| ..| ..| 3.47 |
|774|2075| P | Coch. | 8 | 1431| P |W. Legh. (R)| 0 | 6| 1| 1| ..| 1| ..| ..| ..| ..| ..| ..| 0.78 |
| +---+---+---+---+---+---+---+---+---+---+---+--------+
| Totals (111) | 6| 2| 6| 31| 27| 24| 10| 3| 0| 2| 0| 3.91 |
+-------------------------------------------------------------+---+---+---+---+---+---+---+---+---+---+---+--------+
| B. DARK BRAHMA CROSSES. |
+---+----+----+-------------+----+-----+----+------------+----+---+---+---+---+---+---+---+---+---+---+---+--------+
|727| Y| P | D. Br. | 10 | 381| P | Houd. | 0 | ..| ..| ..| ..| 2| 3| 2| 1| 2| ..| ..| 5.80 |
|727| 121| P | Do. | 10 | 381| P | Do. | 0 | 1| ..| ..| 1| 1| 5| 4| ..| ..| ..| ..| 4.67 |
|823|2030| P | Do. | 7 | 3858| P | M × P | 0 | ..| ..| 5| 16| 15| 4| 1| 2| ..| ..| ..| 3.67 |
|823| Y| P | Do. | 8 | 3858| P | Do. | 0 | ..| ..| 1| 7| 6| 2| ..| ..| ..| ..| ..| 3.56 |
|838|3814| P | W. Legh. | 0 | 122| P | D. Br. | 6 | ..| 2| 2| 6| 6| 1| 1| ..| ..| ..| ..| 3.28 |
|838| 202| P | Min. | 0 | 122| P | Do. | 6 | ..| ..| 2| 5| 3| ..| ..| ..| ..| ..| ..| 3.10 |
|838| 71| P | W. Legh. | 0 | 122| P | Do. | 6 | ..| ..| ..| 1| ..| ..| ..| ..| ..| ..| ..| 3.00 |
|838|3832| P | Do. | 0 | 122| P | Do. | 6 | 1| 1| ..| 1| 1| 2| ..| ..| ..| ..| ..| 3.00 |
|838| 10| P | Do. | 0 | 122| P | Do. | 6 | ..| 1| ..| 3| 1| ..| ..| ..| ..| ..| ..| 2.80 |
|816| 121| P | D. Br. | 9 | 4912| P | M × P | 0 | ..| ..| 8| 4| 1| 1| ..| ..| ..| ..| ..| 2.64 |
|816|5838| P | Do. | 9 | 4912| P | Do. | 0 | ..| ..| 5| 5| 1| ..| ..| ..| ..| ..| ..| 2.64 |
|838|5418| P | W. L., Min. | 0 | 122| P | D. Br. | 6 | 1| 1| 3| 3| 1| 1| ..| ..| ..| ..| ..| 2.50 |
|816|5979| P | D. Br. | 6 | 4912| P | M × P | 0 | 4| 3| 4| 7| 4| 1| 1| ..| ..| ..| ..| 2.46 |
|816|2353| P | Do. | 5 | 4912| P | Do. | 0 | ..| 2| 2| 4| 1| ..| ..| ..| ..| ..| ..| 2.44 |
|816|5977| P | Do. | 4 | 4912| P | Do. | 0 | ..| 3| 2| 1| ..| 1| ..| ..| ..| ..| ..| 2.14 |
|816|5835| P | Do. | 5 | 4912| P | Do. | 0 | 3| 5| 5| 8| 3| ..| ..| ..| ..| ..| ..| 2.12 |
|816|5840| P | Do. | 5 | 4912| P | Do. | 0 | 5| 1| 3| 4| 1| ..| ..| ..| ..| ..| ..| 1.64 |
|823|6626| P | Do. | 2 | 3858| P | Do. | 0 | 1| 10| 2| 2| ..| ..| ..| ..| ..| ..| ..| 1.33 |
|816|5980| P | Do. | 5 | 4912| P | Do. | 0 | 5| 8| 1| 5| ..| ..| ..| ..| ..| ..| ..| 1.32 |
| +---+---+---+---+---+---+---+---+---+---+---+--------+
| Totals (268) | 21| 37| 45| 83| 47| 21| 9| 3| 2| 0| 0| 2.84 |
+-------------------------------------------------------------+---+---+---+---+---+---+---+---+---+---+---+--------+
| C. SILKIE CROSSES. |
+---+----+----+-------------+----+-----+----+------------+----+---+---+---+---+---+---+---+---+---+---+---+--------+
|774| 777| P | Silkie. | 8 | 1176| P | W. Legh. | 0 | 3| ..| 1| 1| 1| ..| ..| ..| ..| ..| ..| 1.50 |
|744| 681| P | Do. | 5 | 1176| P | Do. | 0 | 11| 2| 1| 1| 1| 1| ..| ..| ..| ..| ..| 0.94 |
|744| 469| P | Do. | 1 | 1176| P | Do. | 0 | 11| 3| ..| ..| ..| ..| ..| ..| ..| ..| ..| 0.21 |
| +---+---+---+---+---+---+---+---+---+---+---+--------
| Totals (37) | 25| 5| 2| 2| 2| 1| 0| 0| 0| 0| 0| 0.76 |
+-------------------------------------------------------------+---+---+---+---+---+---+---+---+---+---+---+--------+
| SUMMARY. |
+---------------------------------------+---------------------------------------------------------------------------
| | Grades of boot in offspring, reduced to percentages. |
| Crosses. +-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+--------+
| | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 |Average.|
+---------------------------------------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+--------+
|Cochin | 5.4| 1.8| 5.4| 28.0| 24.3| 21.6| 9.0| 2.7| 0.0| 1.8| ..| 3.91 |
|Brahma | 7.8| 13.8| 16.8| 31.0| 17.5| 7.8| 3.4| 1.1| 0.7| ..| ..| 2.84 |
|Silkie | 67.6| 13.5| 5.4| 5.4| 5.4| 2.7| ..| ..| ..| ..| ..| 0.76 |
+---------------------------------------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+--------+
An inspection of Table 33, which gives the distribution of grades of
boot in the offspring constituting the first hybrid generation, might
well lead to the conclusion that inheritance is here of a blending
nature, or that, if either condition is dominant, it is the booted
one, as suggested in my report of 1906. On this hypothesis the
offspring with no boot illustrate imperfection of dominance, and one
would say that, in booting, dominance is very imperfect.
However plausible such an interpretation might appear when based
on the first hybrid generation alone, it becomes untenable when
subsequent generations are taken into account, as we shall see
later. The hypothesis breaks down completely in the second hybrid
generation and we are forced to the opposite hypothesis, namely, that
the clean-shanked condition is dominant. Such an hypothesis would
seem, at first, to contravene the principle enunciated in my report
of 1906 that the more progressive condition is dominant over the
less progressive condition, or absence. But such is not necessarily
the fact. We have no right to assume that presence of boot is the
new character. The rest of the body of poultry (save the head) is
covered with feathers. If the foot is not it must be because there
is something in the skin of the foot that inhibits the development
of feathers there. And this inhibiting factor is dominant over its
absence.
Table 33 shows that the Silkie crosses yield an exceptionally high
per cent of the dominant clear-footed condition. This is additional
evidence that the Silkies are DR, and so this cross produces 50
per cent of pure extracted dominants in addition to 50 per cent of
heterozygotes in booting.
To get further light on the nature of inheritance of booting we pass
to the examination of the second hybrid generation (table 34).
In the case of Silkies, which throw 67.6 per cent clean-shanked
progeny in F1, we find in F2 only about 60 per cent clean-shanked.
This diminution is, of course, due to the extraction of some pure
booted recessives, which draw from the proportion of clean shanks.
In the case of the Cochins and Dark Brahmas, expectation, with
perfect dominance, is that 75 per cent of the offspring shall
be clean-shanked. Since dominance is imperfect (as shown by the
occurrence of many booted birds in F1) we should look for an actual
failure to reach so large a proportion, but we are hardly prepared
for the result that in most of the F2 crosses of Cochins and Brahmas
less than 25 per cent of the offspring are clean-shanked. In 4 pens
the average is only 10 to 12 per cent, and in one only 2 per cent
of the offspring fail to develop feathers on the feet. What shall
we say of such a case as the last? The history of the father (No.
666) is absolutely certain; his mother was No. 121, the original
Dark Brahma female, with a boot of grade 9 and a record in her
immediate progeny that indicates perfect purity of booting in her
germ-cells. His father was a White Leghorn with clean shanks and
without a suspicion of having such antipodal blood as the Asiatic
in his ancestry. No. 666 is certainly heterozygous in boot, if
boot is a single unit. The hens with which No. 666 were mated were
clearly heterozygous, as is known not only from their ancestry, but
also from their behavior when mated with another cock, No. 254, in
which case they threw 12 per cent non-booted offspring. If now both
parents are heterozygous they must produce 25 per cent recessives.
This is the fact that forces us to conclude that clean shank is not
recessive, but dominant and due to an inhibitor that frequently
_fails to dominate_. In table 31 the two recessive varieties, mated
_inter se_, produce no featherless shanks; the feathers grow freely
as they do over the rest of the body. Some of the Silkies of table
31, however, are really heterozygous, with the dominant inhibitor not
showing; consequently they throw a large proportion of non-booted
offspring. In F1, as table 33 shows, the heterozygous offspring
have a reduced boot and perfect dominance--complete inhibition of
boot--in from 6 to 68 per cent. Dominance is most complete in the
Silkies, where, the feathering being feeble, the inhibitor has, as
it were, less to do in overcoming it. In F2 the expected 75 per cent
dominant is approached in the case of the Silkies (62 per cent and
59 per cent, respectively), but inhibition is very imperfect in the
Cochin and Brahma crosses, being reduced to between 25 and 2 per
cent. More proof that boot is due to the absence of a factor rather
than to its presence is found in this generation. If absence of boot
is recessive, then, combined with imperfection of dominance, _at
least_ 25 per cent of the offspring should be recessive and probably
a much larger proportion. The results in table 34 are absolutely
incompatible with this hypothesis, since, in one case, there are only
2 per cent that can not develop boot. Two extracted clean-footed
birds sometimes throw boot and sometimes not, and this result is to
be expected on the hypothesis that clean-footedness is dominant, but
two heavily booted birds can not transmit the boot inhibitor.
TABLE 34.--_Distribution of boot-grade in the F2 generation of
booted × non-booted poultry._
+------------------------------------------------------------------------------------------------------------------+
| COCHIN CROSSES. |
+---+---------------------------------------+-------------------------------------+--------------------------------+
| | Mother. | Father. | Offspring. |
+---+---+-----+-----------------------------+---+----+---------------------+------+--------+-------+-------+-------+
|Pen|No.| Gen.| Races. | Grade.|No.|Gen.| Races. |Grade.| Boot | Boot | Boot | P. ct.|
|No.| | | | | | | | |present.|slight.|absent.|absent.|
+---+---+-----+---------------------+-------+---+----+-----+---------------+------+--------+-------+-------+-------+
|650|170| F1 |Bl. Coch. × Wh. Legh.| Pr. |265| F1 |Bl. Coch. × Wh. Legh.| Pr. | 19 | 2 | 2 | 8.7 |
|650|263| F1 | Do. | Pr. |265| F1 | Do. | Pr. | 36 | 2 | 2 | 5.0 |
|650|278| F1 | Do. | Pr. |265| F1 | Do. | Pr. | 26 | 4 | 4 | 11.8 |
|650|361| F1 | Do. | Pr. |265| F1 | Do. | Pr. | 24 | 2 | 9 | 25.7 |
|650|364| F1 | Do. | Pr. |265| F1 | Do. | Pr. | 39 | 5 | 3 | 6.4 |
| | | | +--------+-------+-------+-------+
| | | | Totals (179) | 144 | 15 | 20 | 11.1 |
| | | | | | | | |
| | | | |========|=======|=======|=======|
|654|602| F1 |Wh. Legh. × Bf. Coch.| Pr. |704| F1 |Wh. Legh. × Bf. Coch.| Pr. | 11 | 4 | 5 | 25.0 |
|654|828| F1 | Do. | Pr. |704| F1 | Do. | Pr. | 7 | 11 | 0 | 0.0 |
|654|640| F1 | Do. | Pr. |704| F1 | Do. | Pr. | 13 | 2 | 3 | 16.7 |
|654|696| F1 | Do. | Pr. |704| F1 | Do. | Pr. | 8 | 5 | 8 | 38.1 |
|654|767| F1 | Do. | Pr. |704| F1 | Do. | Pr. | 3 | 1 | 3 | 42.9 |
|654|697| F1 | Do. | Pr. |704| F1 | Do. | Pr. | 4 | 3 | 6 | 46.2 |
| | | | +--------+-------+-------+-------+
| | | | Totals (97) | 46 | 26 | 25 | 25.8 |
+---+---+-----+-------------------------------------------------------------------+--------+-------+-------+-------+
TABLE 34.--_Distribution of boot-grade in the F2 generation of
booted × non-booted poultry_--Continued.
+-------------------------------------------------------------------------------------------------------------------+
| DARK BRAHMA CROSSES. |
+---+---------------------------------------+-------------------------------------+---------------------------------+
| | Mother. | Father. | Offspring. |
|Pen+----+-----+---------------------+------+----+-----+------------------+-------+--------+-------+-------+--------+
|No.| No.| Gen.| Races. |Grade.| No.| Gen.| Races. |Grade. |Boot |Boot |Boot |P. ct. |
| | | | | | | | | |present.|slight.|absent.|absent. |
+---+----+-----+---------------------+------+----+----+-------------------+-------+--------+-------+-------+--------+
|608| 384| F1 |Wh. Legh. × Dk. Brah.| Pr. |409 |F1|Wh. Legh. × Dk. Brah.|Pr. | 36 | 5 | 3 | 6.8 |
|608| 248| F1 | Do. | Pr. |409 |F1| Do. |Pr. | 32 | 5 | 4 | 9.8 |
|608| 249| F1 | Do. | Pr. |409 |F1| Do. |Pr. | 39 | 11 | 13 | 20.6 |
|608| 395| F1 | Do. | Pr. |409 |F1| Do. |Pr. | 20 | 11 | 10 | 24.4 |
|608| 385| F1 | Do. | Pr. |409 |F1| Do. |Pr. | 20 | 6 | 14 | 35.0 |
| | | | +--------+-------+-------+--------+
| | | | Totals (229) | 147 | 38 | 44 | 19.2 |
| | | | +========+=======+=======+========+
|659| 762| F1 |Wh. Legh. × Dk. Brah.| Pr. |375 |F1|Wh. Legh. × Dk. Brah.|Pr. | 18 | 4 | 1 | 4.4 |
|659| 503| F1 | Do. | Pr. |375 |F1| Do. |Pr. | 23 | 6 | 2 | 6.5 |
|659| 382| F1 | Do. | Pr. |375 |F1| Do. |Pr. | 10 | 2 | 1 | 7.7 |
|659| 250| F1 | Do. | Pr. |375 |F1| Do. |Pr. | 33 | 7 | 5 | 11.1 |
|659| 737| F1 | Do. | Pr. |375 |F1| Do. Pr. | 19 | 2 | 3 | 12.5 |
|659| 387| F1 | Do. | Pr. |375 |F1| Do. Pr. | 16 | 6 | 4 | 15.4 |
| | | | +--------+-------+-------+--------+
| | | | Totals (162) | 119 | 27 | 16 | 9.9 |
| | | | +========+=======+=======+========+
|655| 720| F1 |Wh. Legh. × Dk. Brah.| Pr. |666 |F1|Wh. Legh. × Dk. Brah.|Pr. | 5 | 2 | .. | 0.0 |
|655| 724| F1 | Do. | Pr. |666 |F1| Do. |Pr. | 6 | 1 | .. | 0.0 |
|655| 728| F1 | Do. | Pr. |666 |F1| Do. |Pr. | 3 | 1 | .. | 0.0 |
|655| 730| F1 | Do. | Pr. |666 |F1| Do. |Pr. | 4 | .. | .. | 0.0 |
|655| 732| F1 | Do. | Pr. |666 |F1| Do. |Pr. | 9 | .. | .. | 0.0 |
|655| 734| F1 | Do. | Pr. |666 |F1| Do. |Pr. | 3 | .. | .. | 0.0 |
|655| 761| F1 | Do. | Pr. |666 |F1| Do. |Pr. | 6 | 2 | .. | 0.0 |
|655| 800| F1 | Do. | Pr. |666 |F1| Do. |Pr. | 1 | .. | .. | 0.0 |
|655| 721| F1 | Do. | Pr. |666 |F1| Do. |Pr. | 9 | 1 | 1 | 9.1 |
| | | | +--------+-------+-------+--------+
| | | | Totals (54) | 46 | 7 | 1 | 1.9 |
| | | | +========+=======+=======+========+
|655| 724| F1 |Wh. Legh. × Dk. Brah.| Pr. |254 |F1|Wh. Legh. × Dk. Brah.|Pr. | 3 | .. | .. | 0.0 |
|655| 734| F1 | Do.
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