When two sex-linked characters are involved in a cross it is possible to determine by suitable matings whether an interchange between the chromosomes that bear them has taken place. In the case of the sex chromosomes only one sex, the male, has both like chromosomes, viz, ZZ, and we expect from analogy with the Drosophila work that crossing-over would be found between the sex chromosomes only in the male. Goodale has recently (1917) made the important discovery that in poultry crossing-over takes place between the sex chromosomes (ZZ) in the male, but not in the female (ZW or ZO). This relation, therefore, is the reverse in birds and flies, for, in the one, crossing-over takes place in the female and in the other in the male. Whether this difference extends also to the other chromosomes in birds as it does in flies is as yet not known.
Several years ago some crosses between gold and silver Campines were reported by Rev. E. Lewis Jones. The results are consistent with the view that a sex-linked factor pair is responsible for this difference in color, although the author does not apply this view to his results. The results may be seen in the table on page 16, to which Jones has prefixed the number of individuals. The cross also involved hen-feathering versus cock-feathering, which appears here (as in other cases) to be a non-sex-linked dominant factor. As stated above there are in the results a few apparent inconsistencies with this interpretation, due possibly to heterozygous females having been used in the crosses.
Lefevre crossed Silver Spangled Hamburgs and Brown Leghorns. The spangling was found to be a sex-linked dominant factor. A spangled cock bred to a Leghorn hen gives spangled sons and daughters; a spangled hen by a Leghorn male gave spangled sons and not spangled daughters. The daughters do not transmit spangling. Other factors may obscure the results, especially factors for black, or the localization of the pattern. Lefevre says “it would seem probable that multiple factors for black, introduced by the Brown Leghorns, are present, and that these factors may have a cumulative effect, with the result that pigmentation is developed to varying degrees of extension.” Whether the factors for black spoken of as coming from the Leghorns are dominant wild-type factors that have mutant allelomorphs in the Silver Spangled Hamburg is not entirely clear from the quotation.
Baur gives in his Introduction to the Study of Heredity (1914, pp. 202-203) some results (unpublished) that Hagedoorn had obtained by crossing gold and silver races of Assendelver birds. The factor is sex-linked and is no doubt the same factor reported by Jones for gold and silver Campines and by Sturtevant for Columbian Wyandottes. Silver dominates gold and the sex relations are the same as those already reported by others for poultry, viz, the male is ZZ, the female ZW. Gold hens by a heterozygous silver[6] gave 162 silver cocks, 163 silver hens, 168 gold cocks, 160 gold hens, expressed graphically (g for gold, s for silver):
| Zᵍ—W♀ × Z—Zᵍ♂ | |||
| ZˢZᵍ—ZᵍZᵍ—ZˢW—ZᵍW | |||
| Silver | Gold | Silver | Gold |
| male | male | female | female |
When a silver hen was united to a gold cock there were 246 silver cocks and 243 gold hens—crisscross inheritance.
Summary.
From the standpoint of the Brown Leghorn type representing the wild type, the following colors and patterns represent dominant mutations from that type:
| Dominants. | |
| White of White Leghorn. Silver of Dark Brahma. Black of Minorca. Lacing of Brahma. | Barring of Plymouth Rock. Black (?) of Plymouth Rock. Buff (or red). |