Lippincott has recently studied the Andalusian cross and obtained essentially the same results as his predecessors. He calls attention to an interesting fact in the splashed whites, namely, that the color splashes are blue when they are found in those parts of the body where the color is blue in the Andalusian. Although the Andalusian is always spoken of as a blue bird, the hen only is entirely blue, while the male is black above and blue below. The splashes on a white male correspond to the black and blue of the Andalusian male, and are black if above and blue if below.
Lippincott found also that the blue birds differ from the black in two characteristics, viz, in the blues the pigment is in larger masses, i. e., it is more clumped, leaving more white between the clumps than in the blacks, and in the blues the pigment is absent in the extremities of the barbules. If the clumping and the condition of the barbules are treated as separate entities, each gives a 3:1 ratio. Lippincott concludes, therefore, that the Andalusian cross is a 2-factor case. If each of these characteristics was independent of the other in the sense that some birds had clumped pigment and others deficiencies in the barbules, then one might conclude that he was dealing with a 2-factor case; but if these two characters are only different aspects of the same gene, and when one is present the other is also, the situation is not different from those that are very common, viz, two or more effects produced by the same genetic factor.
Davenport has recorded results of crossing several breeds of different colors (1906 and 1909). The white of the Leghorn was found dominant to the black of the Minorca breed, although the hybrids, “at least the females,” had some black feathers. This white was also found to be dominant to the mottled Houdan and to the “Red-backed game.” On the other hand, a male Tosa with wild-type plumage by recessive White Cochin female gave “barred” males in F₁; the barring coming in, no doubt, from the Cochin and although not at the time recognized by Davenport as sex-linked inheritance, the statement that barring is “associated with maleness” (as already pointed out by Darwin) indicated that the barring that appeared within the cross was probably the sex-linked barring shown by other breeds.
In Davenport’s cross of White Leghorn by Minorca two blues appeared (as stated above), indicating that the same factors were here present that in the Andalusian white and black strain gives the same result,[3] but why only some of the F₁ appear as blue, while others are not blue, is not yet made clear, unless two factors for white were present. White of the Leghorn breed was found not to be as completely dominant over buff as over black. Black was found dominant over the wild-type (Black-Breasted game), but red is present in F₁ birds also to some extent in those places where red is found in the game. Lacing, as shown by the Dark Brahma, is dominant to the plumage of the Tosa. Penciling also is said to be dominant, as shown in females of the cross between the Dark Brahma and Tosa fowl.
In his later paper (1909) Davenport gives fuller information in regard to some of the F₁ cases reported in his first paper, as well as the F₂ results. Thus, in the cross of Silky to Minorca, that gives black F₁ birds, the F₂ count gave 210 black, 57 game, and 95 white—approximately the expectation for two pairs of factors, one of them giving white (9:3:4). Silky by White Leghorn gave white F₁’s, but the males developed red on the wing bow and saddle when they became mature, and the female a faint blush of salmon (“red”) on the breast. In F₂ there were whites, games, and blacks, approximating to expectation for three pairs of factors, one being a dominant white (52:9:3). Silky by Buff Cochin gave a washed-out buff, but with the jungle coloration partly developed in the tail (black) and hackles and wing bow (redder buff). Davenport represents the Buff Cochin as having lost the jungle patterns and coloration, while the Silky retains it. The heterozygous condition of the genes for the wild-type color in F₁ is made responsible for the part development of color. The White Silky is represented as carrying the factor for black (N), hence in F₂ both black and game-colored birds are expected and they were obtained. When Black Cochin is crossed to Buff Cochin, the F₁ males are in general like the game (black and red) while the females are black (except for some red on the hackle). In this case Davenport represents the Black Cochin as showing a factor for jungle-fowl pattern, but lacking the color that is assumed in his other formulæ to go with this pattern. What is meant by this change is not quite clear to me, unless Davenport supposes there is an independent factor for the jungle-fowl pattern which may be filled in by other colors determined by other factors. But were there enough F₁ birds to exclude the possibility that jungle-fowl birds would not appear in this cross?
Davenport has reported a cross between a female White Cochin and a male Tosa (wild type) from which the daughters were Tosa, except that the shafting was broadened, and the saddle feathers and proximal secondaries were obscurely barred (black and buff); the sons were also like the Tosa, but every feather was repeatedly barred (see above). In F₂ there were 15 white, 25 game, and 16 barred birds. Davenport concludes that “barring is clearly heterozygous and confined to the male sex,” and in a footnote he adds that the sex-linked barring factor of the Plymouth Rock is different from that of this Cochin-Tosa cross, but Goodale informs me that the barring that appeared in this cross is probably the same as that in Barred Rocks.
As pointed out, an interesting feature of color inheritance in poultry is the large number of cases of sex-linked inheritance. It might seem probable here, as in the case of Drosophila, that this is due to a well-recognized difference between sex-linked and autosomal characters, namely, that a recessive mutation in one of the sex chromosomes of a sperm-cell of the male bird will have a chance of showing its effect immediately if that sperm-cell unites with an egg without a Z to form a daughter, whereas it would not immediately show up in the offspring if the mutation were autosomal.[4] In consequence the recessive mutant would have a greater chance of being observed and selected if it appeared in a sex chromosome. But dominant sex-linked characters, however, have the same chance as dominant autosomal ones and the question turns therefore on the kinds of characters shown in the cross.
The first indication of sex-linkage in fowls was furnished by evidence that Spillman published in 1903 on information supplied by poultry-men—information that has been proven subsequently to have been accurate. Spillman pointed out clearly the similarity between the facts he quoted and the then known cases of sex-linkage in the canary and in the currant moth. The case referred to by Spillman was a cross between Barred Plymouth Rock and Black Langshan. Goodale and I repeated the cross, using both Plymouth Rock and American Dominques, publishing the results in 1912. In addition to the F₁ results evidence was obtained for the F₂ generation. The theory was also tested by back-crossing. The results of such a cross that are typical for all cases of the sort are briefly as follows: Plymouth Rock cock by Langshan hen gives F₁ barred sons and barred daughters. These inbred give F₂ barred cocks and barred and black hens (2:1:1).
In the following schemes the sex chromosomes are represented by Z and W, while the exponents stand for the factors involved, viz, B for barred and b for not-barred, which here means a black bird.
Barred♂ Black♀
P₁ ZᴮZᴮ ZᵇbW
\ /
\ /
\ /
\ /
ZᴮZ ZᴮW
F₁ Barred♂ Barred♀
\ /
\ /
ZᴮZᴮ ZᴮZᵇ ZᴮW ZᵇW
Barred♂ Barred♂ Barred♀ Black♀