CHAPTER X.

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._

+------------+-----------+-----------+-----------+ | C2J2N2 N | C2J2Nn N | CcJ2N2 N | CcJ2Nn N | | C2J2Nn N | C2J2n2 G | CcJ2Nn N | CcJ2n2 G | | CcJ2N2 N | CcJ2Nn N | c2J2N2 W | c2J2Nn W | | CcJ2Nn N | CcJ2n2 G | c2J2Nn W | c2J2n2 W | +------------+-----------+-----------+-----------+

TABLE 58.

+---------+-----------------------+-----------------------+-----------------------+ | | Black. | White. | Game. | | Pen No. +-----------+-----------+-----------+-----------+-----------+-----------+ | | Observed. | Expected. | Observed. | Expected. | Observed. | Expected. | +---------+-----------+-----------+-----------+-----------+-----------+-----------+ | 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._

+-----------------+------+------+------+------+ | Zygotic formula.|White.|White | Game.|Black.| | | |+ 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. +---------------+-------------+ | | | Calculated. | Observed. | +------------+-----------------+---------------+-------------+ | | | | | | 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._

+-----------------+------+------+-------+-------+ | Zygotic |White.|Buff. |Buff + | Game. | | formula. | | |black. | | +-----------------+------+------+-------+-------+ | 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. | | | +--------------+-----------+ | | | Calculated. | Observed. | +-------------------+-----------+--------------+-----------+ | 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.