CHAPTER X._Inheritance of Characteristics in Domestic Fowl

PLUMAGE COLOR.

A. THE GAMETIC COMPOSITION OF THE VARIOUS RACES.

Plumage color, like hair color, varies greatly among domesticated

animals. This diversity is, no doubt, in part due to the striking

nature of color variations, but chiefly to the fact that the

requisite variations are afforded in abundance. The principal color

varieties, in poultry as in other domesticated animals, are melanism,

xanthism, and albinism. In addition, poultry show the dominant white,

or "gray" white, first recognized in poultry by Bateson and Saunders

(1902), which is also found in many mammals, as, for instance, in

goats, sheep, and cattle. Besides these uniform colors, we find

numerous special feather-patterns, such as lacing (or edging of the

feather), barring, penciling, and spangling. Also, there are special

patterns in the plumage as a whole, such as wing-bar, hackle, saddle,

breast, and top of head (crest). Now, all of these color characters

are inherited each in its own definite fashion.

In studying the color varieties of poultry we must first of all, as

in flower color (Correns, 1902), mice (Cuénot, 1903), guinea-pigs

and rabbits (Castle), various plants and animals (Bateson and his

pupils), recognize the existence of certain "factors." In poultry the

factors that I have determined are as follows:

_C_, the color factor, absence of which results in albinism.

_J_, the Jungle-fowl pattern and coloration.

_N_ (nigrum), the supermelanic factor.

_X_, the superxanthic or "buff" factor.

_W_, the graying (white) factor.

We have now to consider how these factors are combined in birds of

the different races.

1. WHITE.

_Albinos._--These seem to be of two different origins:[9] White

Cochins and white Silkies. The white Silkies that I have studied

have the gametic formula _cJnwx_; _i. e._, they have the Jungle-fowl

marking, but lack the "color enzyme," supermelanic coat, the graying

factor, and the xanthic factor.

[9] Bateson and Punnett (1908, p. 28) recognize three

"kinds" of recessive whites--that of the Silkie, that of the

Rose-comb bantams, and that of "white birds that have arisen in

the course of our experiments." White Cochins have perhaps been

one of the ancestors of Rose-comb bantams; Bateson's new white

lay recessive in the White Dorking and when mated to the White

Silkie throws Game-colored offspring.

"_Grays._"--White Leghorns and their derivatives belong to this

class. Its gametic formula is: _CJNWx_. This indicates that the race

contains the color enzyme, as well as the Jungle pattern and the

supermelanic coat. But all of these are rendered invisible by the

graying factor _W_. The superxanthic factor is missing.

2. BLACK.

The uniform black birds that I have studied are of several sorts.

The Black Minorca and White-faced Black Spanish have the gametic

formula _CJNwx_. Owing to the absence of the graying factor and the

presence of the color factor these appear as pigmented birds, but the

supermelanic coat, _N_, obscures the Jungle coloration, so that the

bird appears entirely black. Nevertheless the black is not of uniform

quality, but just those parts of the feathers of the wing, back,

hackle, saddle, and breast that are red in the Jungle fowl are of an

iridescent black, while the portion that is not red in the Jungle is

of a dead black.

The Black Cochin has the gametic formula _CINwx_. This differs from

the formula of the Minorca only in this respect: the Jungle pattern

is present, but not the pigmentation that is usually associated with

it.

The Black Game ("Black Devil") that I used in a few experiments

seemed to have the same gametic formula as the Minorca, only the

supermelanic coat was less dense.

3. BUFF.

For this color I used Buff Cochins, the original buff race. The

gametic formula of this race proves to be _CjnwX_--the Jungle-fowl

pattern being absent.

B. EVIDENCE.

The evidence for the gametic interpretations of the self-colored fowl

is derived from hybridizations. It will now be presented in detail.

1. SILKIE × MINORCA (OR SPANISH).

(Plates 3 to 6.)

By hypothesis this cross is between _cJnwx_ and _CJNwx_. The first

generation should give the zygotic formula _CcJ2Nnw2x2_, or, more

simply, _CcJ2Nn_. This formula resembles closely that for the

Minorca; but it differs in this important respect, that the coloring

factor and the supermelanic factor are both heterozygous, and hence

diluted.

Actually I found, as Darwin (1876) did, that the chicks of this

first hybrid generation were all wholly black. In this respect

they differed markedly from the chicks of the Silkie, which are

pure white, and also from the chicks of the Minorca, which are

prevailingly black, but have white belly and outer primaries. The

white in the young chicks of Minorcas is extremely variable in

amount, but never wholly absent; in time, as the bird grows older, it

is replaced by black, so that the adult male and female Minorcas have

a wholly black plumage. The reason for the precocious development of

black pigment over the belly and primaries of the hybrid chicks is

probably the presence of an extension factor (_cf._ Castle, 1909)

derived from the Silkie. Certain it is that the ordinary Jungle

pattern develops pigment on the belly and on the wings, as well as

on other parts of the plumage. The hybrid chicks may be said to have

the extended pigmentation dominant over interrupted pigmentation. In

the adult hybrids a difference appears between the coloration of the

male and female, even as Darwin pointed out. For the latter retains

its uniform blackness, while the former gains red on the wing-bar,

and saddle and hackle lacing (plate 4). Now, since all the factors

present in the Minorca, and none others, are present in the hybrids,

why should the male hybrids show red, and why should the males show

red and not the females? The answer to the first question is, I

think, clear. While the Jungle pattern of black and red is completely

obscured by the undiluted _N_ factor of the Minorca, it is only

incompletely covered by the diluted, heterozygous _N_ factor of the

hybrid. Hence the red appears in greatly reduced amount, as compared

with the Jungle-fowl. In the female Jungle-fowl there is little red

and consequently none shows in the female hybrid. Thus the difference

in the sexes of the hybrids corresponds to the sexual dimorphism of

the Jungle-fowl; but the hybrids are, as indicated, very unlike the

Jungle-fowl in coloration (_cf._ plates 1 and 2).

Since segregation takes place in the gametes of these heterozygotes,

4 kinds of gametes are possible, namely, _CJN_, _CJn_, _cJN_, _cJn_.

On mating heterozygotes together, zygotes of 16 types will be formed,

as in table 57.

TABLE 57.--_Zygotes in F2 of Silkie × Minorca hybrids and their

corresponding somatic colors._

+------------+-----------+-----------+-----------+

+------------+-----------+-----------+-----------+

TABLE 58.

+---------+-----------------------+-----------------------+-----------------------+

| Pen No. +-----------+-----------+-----------+-----------+-----------+-----------+

+---------+-----------+-----------+-----------+-----------+-----------+-----------+

| 709 | 119 | 116 | 55 | 51 | 31 | 38 |

| 804 | 91 | 89 | 40 | 39 | 26 | 29 |

| +-----------+-----------+-----------+-----------+-----------+-----------+

| Total. | 210 | 205 | 95 | 90 | 57 | 67 |

+---------+-----------+-----------+-----------+-----------+-----------+-----------+

In the foregoing table there is given after each combination a

letter: _N_ standing for black, the appearance of the soma; _G_

standing for Game-colored, and _W_ standing for white. No distinction

is made between pure blacks and those that, while black as chicks,

subsequently show some red in the male. Such a distinction was

impracticable because most of the color determinations are made

on the young chicks. It appears that in 16 progeny expectation is

9 black, 4 white, and 3 Game-colored. Actually 362 offspring were

obtained, with the results shown in table 58. Nothing is more

striking than to see the hens of this F2 generation with evidences of

the female Game pattern (plate 6).

Comparing observed results in the distribution of colors in the F2

generation with expectation, it is seen that the proportions are

close, and this closeness of observation with expectation is evidence

for the correctness of the hypothesis.

The hypothesis may be further tested in later generations by breeding

together the different sorts of individuals obtained in F2. In

pursuance of such a test I mated various pure black hens with pure

black cocks and those of F1, and, as was to have been expected,

obtained families of different sorts, simply because even pure blacks

have differing gametic constitutions. Thus in pen 824 I mated an

extracted black cock with 3 black hens. All were apparently of the

zygotic constitution _C2J2Nn_, forming gametes _CJN_ and _CJn_. Mated

together these should give the three black combinations _C2J2N2_,

_C2J2Nn_, _C2J2nN_, to one Game, _C2J2n2_. Actually there were

obtained 64 black and 23 Game, 66 to 22 being expectation. In another

pen (pen 804) an F1 cock was mated to various black F2 hens. The

families fall into 2 classes. The cock, of course, produced gametes

_CJN_, _CJn_, _cJN_, _cJn_. With four females like him (Nos. 3902,

3908, 5431, 6043) I got: black 40, white 13, Game 14; expected,

black 38, white 17, Game 13. Three females (Nos. 4715, 4716, 5099)

evidently produced gametes _CJN_, _CJn_. Expectation is that blacks

and Games shall be produced in the proportions of 3 to 1. Actually

30:14 were obtained where 33:11 was expected. All of these results

accord closely with the hypothesis.

The whites obtained in F2 are of 3 types, but in all alike the color

factor is missing. Hence it can not reappear in the offspring, and,

consequently, no colored offspring are to be expected. But, first,

it must be stated that the extracted whites of the F2 generation

are not always of a pure white. Indeed, the parent Silkies are in

some cases not perfectly white, but show traces of "smoke." There

are different degrees of albinism; the coloring enzyme may be absent

to small traces. This variability in degree of albinism is familiar

to all students of albinism in man. My breeding of extracted whites

was done in pen 817 and consisted of a pure white cock (No. 3900)

and 2 hens. Of these 1 (No. 6046) was pure white and produced in a

total of 15 only white offspring, but among those that were described

as unhatched I have recorded traces of pigment in 24 per cent of

the cases. The second hen (No. 3899) had black flecks in the white

plumage. She had 20 offspring, of which 2 (unhatched) are recorded

as having N down, 2 as "blue," and 3 others show traces of black

pigment. Thus, 7 birds in 20, or 35 per cent of all, show more or

less black, even as the albinic mother does. On the whole, however,

omitting from present consideration the phenomenon of incomplete

albinism, we may say that 2 pure albino parents produce only albinic

offspring, while imperfectly albinic parents produce some imperfectly

albinic offspring.

2. SILKIE × WHITE LEGHORN.

By hypothesis this cross is between _cJnwx_ and _CJNWx_. The first

generation should give the zygotic formula _CcJ2NnWwx2_, or, more

simply, _CcJ2NnWw_. This formula resembles closely that of the White

Leghorn, except that the coloring and graying factors and that for

supermelanism are all heterozygous and hence diluted; only the Jungle

coloration remains unchanged. Actually, the first generation yielded

a lot of white birds like the Leghorn, but with this difference,

that, as the males became mature, they gained red on the wing-bar and

to a slight extent on the lacing of the saddle. The females gained a

faint blush of red on the breast. Thus red appeared, in small amount,

in just those places in the respective sexes which are red in the

Jungle-fowl. The explanation of its appearance that I have to suggest

is that, both on account of the diluting of the supermelanic coat and

of the graying factor, the red of the undiluted underlying Jungle

coloration is revealed.

Since the hybrids are heterozygous in respect to 3 pairs of

characters, when segregation occurs each parent produces 8 kinds of

gametes, as follows: _CJNW_, _CJNw_, _CJnW_, _CJnw_, _cJNW_, _cJNw_,

_cJnW_, _cJnw_. When both parents produce these 8 kinds of gametes

we may expect, in 64 offspring, the proportions of the several types

shown in table 59.

TABLE 59.--_Probable frequency in 64 progeny._

+-----------------+------+------+------+------+

| | |+ red.| | |

+-----------------+------+------+------+------+

| | | | | |

| C2J2N2W2 | 1 | .. | .. | .. |

| C2J2N2Ww | 2 | .. | .. | .. |

| C2J2N2w2 | .. | .. | .. | 1 |

| C2J2NnW2 | 2 | .. | .. | .. |

| C2J2NnWw | .. | 4 | .. | .. |

| C2J2Nnw2 | .. | .. | 2 | .. |

| C2J2n2W2 | 1 | .. | .. | .. |

| C2J2n2Ww | .. | 2 | .. | .. |

| C2J2n2w2 | .. | .. | 1 | .. |

| CcJ2N2W2 | 2 | .. | .. | .. |

| CcJ2N2Ww | 4 | .. | .. | .. |

| CcJ2N2w2 | .. | .. | .. | 2 |

| CcJ2NnW2 | 4 | .. | .. | .. |

| CcJ2NnWw | .. | 8 | .. | .. |

| CcJ2Nnw2 | .. | .. | 4 | .. |

| CcJ2n2W2 | 2 | .. | .. | .. |

| CcJ2n2Ww | .. | 4 | .. | .. |

| CcJ2n2w2 | .. | .. | 2 | .. |

| c2J2-- | 16 | .. | .. | .. |

| +------+------+------+------+

| Total (64) | 34 | 18 | 9 | 3 |

+-----------------+------+------+------+------+

While, if the progeny were all to survive to maturity, we might

expect to get the proportions of white and of white-and-red progeny

called for, yet, since the red color appears in most cases at an age

_after_ the chicks are described, it will be necessary in comparing

experience with calculation to combine the first two classes as

whites. We then find the proportions given in table 60.

TABLE 60.

+------------+-----------------+-----------------------------+

| | | In the actual 85 |

| Color. | In 64, | individuals. |

| | calculated. +---------------+-------------+

+------------+-----------------+---------------+-------------+

| | | | |

| White. | 52 | 69 | 68 |

| Game. | 9 | 12 | 16 |

| Black. | 3 | 4 | 1 |

+------------+-----------------+---------------+-------------+

The proportion of whites agrees closely with expectation. If this

is not the case with the other two classes, the discrepancy must

be attributed in part to insufficient observations and in part to

the difficulties of precise classification in the early stages.

The result is so close, however, as to lend strong support to our

hypothesis as to the gametic constitution of the parents. Nothing is

more striking, and to the unprejudiced mind more convincing, than the

appearance of typically Game-colored birds in the grandchildren of

wholly white parents.

3. SILKIE × BUFF COCHIN.

(Plates 7, 8.)

By hypothesis this cross is between _cJnwx_ and _CjnwX_. The first

generation should give the zygotic formula _CcJjn2w2Xx_, or, more

simply, _CcJjXx_. The formula differs much from that of either

parent, and the progeny themselves are no less remarkable. They have

a washed-out buff color (since they are heterozygous in both _C_

and _X_), and the Jungle pattern shows itself in the black tail and

slightly redder buff of the wing-bar and hackles in the male. Since

the hybrids are heterozygous in respect to 3 pairs of characters,

when segregation occurs each parent produces 8 kinds of gametes, as

follows: _CJX_, _CJx_, _CjX_, _Cjx_, _cJX_, _cJx_, _cjX_, _cjx_. In

F2 the types listed in table 61 may be expected in 64 offspring.

TABLE 61.--_Distribution of colors, theoretic classes.--Probable

frequency in 64 progeny._

+-----------------+------+------+-------+-------+

+-----------------+------+------+-------+-------+

| C2J2X2 | .. | .. | 1 | .. |

| C2J2Xx | .. | .. | 2 | .. |

| C2J2x2 | .. | .. | .. | 1 |

| C2JjX2 | .. | .. | 2 | .. |

| C2JjXx | .. | .. | 4 | .. |

| C2Jjx2 | .. | .. | .. | 2 |

| C2j2X2 | .. | 1 | .. | .. |

| C2j2Xx | .. | 2 | .. | .. |

| C2j2x2 | 1 | .. | .. | .. |

| CcJ2X2 | .. | .. | 2 | .. |

| CcJ2Xx | .. | .. | 4 | .. |

| CcJ2x2 | .. | .. | .. | 2 |

| CcJjX2 | .. | .. | 4 | .. |

| CcJjXx | .. | .. | 8 | .. |

| CcJjx2 | .. | .. | .. | 4 |

| Ccj2x2 | .. | 2 | .. | .. |

| Ccj2Xx | .. | 4 | .. | .. |

| Ccj2x2 | 2 | .. | .. | .. |

| c2-- | 16 | .. | .. | .. |

| +------+------+-------+-------+

| Total | 19 | 9 | 27 | 9 |

+-----------------+------+------+-------+-------+

The classification here employed can not be used in detail in

comparing observed results with expectation, for the distinction

between buff and buff-and-black appears only in chicks that have

acquired the permanent plumage. Consequently it will be found

necessary to combine these two classes into one and then make the

comparison--as is done in table 62.

TABLE 62.--_Distribution of colors, combined classes._

+-------------------+-----------+--------------------------+

| Color. | In 64, | In the actual |

| |calculated.| 58 individuals. |

| | +--------------+-----------+

+-------------------+-----------+--------------+-----------+

| Buff (and black).| 36 | 33 | 34 |

| White. | 19 | 17 | 17 |

| Game. | 9 | 8 | 7 |

| +-----------+--------------+-----------+

| Total. | 64 | 58 | 58 |

+-------------------+-----------+--------------+-----------+

The correspondence is certainly close. The hypothesis of factors thus

receives additional support and the variability of the offspring in

the second hybrid generation is sufficiently explained.

4. WHITE LEGHORN × BLACK MINORCA.

As we have already seen, the gametic formula of the White Leghorn is

_CJNWx_ and that of the Minorca is _CJNwx_, so that the F1 generation

has the zygotic formula _C2J2N2Wwx2_ or, more simply, _C2J2N2Ww_.

These heterozygotes are white because of the graying factor, but,

as this factor is diluted, some black shows, particularly in the

females. In F2, on account of there being only 1 heterozygous factor,

only 3 kinds of zygotes are formed, _C2J2N2W2_, _C2J2N2Ww_, and

_C2J2N2w2_, in the proportions 1: 2: 1. Since not only offspring

homozygous in _W_, but also all male heterozygotes, are white

and many female heterozygotes are late in revealing any pigment,

it is necessary to consider together individuals homozygous and

heterozygous in _W_. Consequently we may expect 75 per cent of the

offspring to show white or white-black-speckled plumage, and 25 per

cent black or black and white like the young Minorca. Actually, in

154 offspring (pen 633) I obtained 116 white + white-black + blue,

and 38 black with more or less white and including 4 barred, of which

more later. Expectation is 115.5 and 38.5, respectively.

In another experiment I crossed the F1 hybrids on a pure White

Leghorn and got 41 offspring, all white except 1 that showed some

black specks. All results thus accord with hypothesis.

5. WHITE LEGHORN × BUFF COCHIN.

(Plate 9.)

These two races afford the gametic formulæ _CJNWx_ and _CjnwX_,

respectively. The F1 hybrids consequently have the zygotic formula

_C2JjNnWwXx_. Such hybrids are heterozygous in all factors except

_C_. Such complex heterozygotism, combined with the well-known

sex differences in color of heterozygotes, leads to a very great

diversity of the offspring. As a matter of fact I found, as Hurst

did, that the young were sometimes quite white, sometimes white and

buff, and sometimes showed also a little black. Since there are 4

heterozygous characters, there are 256 possible combinations of

them, which reduce to 81 different kinds of combinations. Owing to

the ambiguous nature of the soma in many of the heterozygotes and to

the relatively small number of offspring, it is useless to compare

theoretical and observed distributions of plumage colors in the

somas. Suffice it to say that white, buff, black, and Game-colored

chicks all appeared in the F2 generation, as well as some with a

mixture of colors, as called for by the hypothesis. White, due to the

powerful graying factor, was the commonest color, buff and black were

about equally common, and each about one-third as abundant as white,

while Games, due to the hypostatic J factor, were about one-third as

common as buff. All this, again, is explicable upon our hypothesis

and upon none other so far proposed. In mating the F2 generation

with each other or with the White Leghorn the result must vary with

the gametic output of the hybrid, which is obviously very different

in different cases. A hen, of a light buff color spangled with white

spots and having a black tail, presumably formed gametes _CJnWX_,

_CJnwX_, _CJNWX_, _CJNwX_. Mated with the White Leghorn, _CJNWx_,

she produced 8 pure whites, 4 whites with some black and red, 2

buff and white, and 3 black with trace of white. Expectation in 16

offspring would be about 4 pure whites, 4 white mixed with pigment, 4

buffs with white (and black?), and 4 blacks mixed with other colors.

This is merely an illustration of the way the confused combinations

of colors become intelligible, and even necessary on the factor

hypothesis.

6. BLACK COCHIN × BUFF COCHIN.

(Plate 10.)

The factors involved in this cross seem to be _CINx_ for the

Black Cochin (in which _I_ stands for the Jungle pattern without

any associated color factor) and _CjnX_ for the Buff Cochin, as

before. The F1 generation has the zygotic composition _C2IjNnXx_,

and the females are all black, except for a variable amount of red

on the hackle, and the males are black and red, like Games. The F2

generation is remarkable. Since 3 factors are heterozygous, there are

64 possible combinations and 27 differing ones. In table 63 is given

a list of these different combinations and of the probable associated

somatic colors. The prefixed number indicates the frequency of each

combination.

TABLE 63.

+----------------------------+----------------------------+----------------------------+

| 1 C2I2N2X2 Black. | 2 C2IiN2X2 Black. | 1 C2i2N2X2 Black. |

| 2 C2I2N2Xx Black. | 4 C2IiN2Xx Black. | 2 C2i2N2Xx Black. |

| 1 C2I2N2x2 Black. | 2 C2IiN2x2 Black. | 1 C2i2N2x2 Black. |

| 2 C2I2NnX2 Black and red.| 4 C2IiNnX2 Black and red.| 2 C2i2NnX2 Black and red.|

| 4 C2I2NnXx Black. | 8 C2IiNnXx Black. | 4 C2i2NnXx Black. |

| 2 C2I2Nnx2 Black. | 4 C2IiNnx2 Black. | 2 C2i2Nnx2 Black. |

| 1 C2I2n2X2 Buff. | 2 C2Iin2X2 Buff. | 1 C2i2n2X2 Buff. |

| 2 C2I2n2Xx Buff. | 4 C2Iin2Xx Buff. | 2 C2i2n2Xx Buff. |

| 1 C2I2n2x2 White. | 2 C2Iin2x2 White. | 1 C2i2n2x2 White. |

+-----------------------------+---------------------------+----------------------------+

Uniting the blacks and black-and-reds (since red appears only in one

sex and often not until late in life) we find the following relation

between the calculated and the observed proportions in 86 offspring:

Calculated, black 65, buff 16, white 5; observed, black 61, buff 17,

white 8.

In still another pen (848) the F2 hybrids were mated to a Buff

Cochin. Only 21 chicks were raised. Expectation is, black 10.4, buff

5.2, white 5.2. Actually there were obtained, black 7, buff 10, white

4. Half of the calculated blacks are really heterozygous in both

black and buff; so expectation is a little uncertain, and probably

should be given as something under 10.4. Also, on account of small

numbers, a close agreement is not to be expected; but calculation and

observation are at least of the same order.