52 Genetic Studies in Poultry 



tation of the silky fowl, as well as that for shank pigmentation, etc. In the 

 present case however it is the dominant character that is transmitted 

 to her daughters by the heterozygous hen, and the recessive to her sons. 

 When we come to the purely henny breeds we might at first sight 

 suppose the factor to be carried by both sex-chromosomes, in the male 

 as well as in female. We are, however, at once met with the difficulty 

 that the heterozygous hen, produced by mating a hen of a normal breed 

 with a henny cock, transmits the character to all of her daughters but 

 to only half of her sons. Such a result challenges comparison with the 

 cases of sex-linked heredity in man where the female carrier, mated with 

 a normal, transmits to half of her sons. The resemblance however is but 

 superficial : in the human case the sexually homozygous female transmits 

 to half of the heterozygous sex, while in poultry it is the sexually hetero- 

 zygous female that transmits the peculiarity to half of the homozygous sex. 

 We feel that the simplest way of summarizing the facts is to suppose 

 that in all poultry the factor associated with the sex-linked mechanism 

 comes into play ; whereas in families in which henny cocks appear, an 

 independent, though apparently identical factor also comes into operation, 

 and that in heredity this factor behaves in the usual way, uncomplicated 

 by sex linkage. The hen of a pure henny strain is, on this view, homo- 

 zygous for A, which can be transmitted to either sex. At the same 

 time she is also heterozygous for the factor A' which she transmits only 

 to her daughters. Again, a hen that arises from mating a pure henny 

 cock with a normal hen is heterozygous for both A and A'. The latter 

 she transmits to her daughters only ; the former to half of her sons and 

 half of her daughters. Half of her sons will be henny. But since all of 

 her daughters receive A' through the sex-linked mechanism, those which 

 receive A will not be visibly distinct from those which do not. The 

 former, however, differ from the latter in transmitting henny feathering 

 to half of their sons. A and A' we regard as producing the same effect, 

 but difi'ering in the mechanism of their transmission. Translated into 

 terms of chromosomes, this amounts to saying that A' is only found 

 in the sex chromosome peculiar to the female S whereas A may occur in 

 some pair of chromosomes which is equally represented in both sexes. 

 Whether the sex-linked connection preceded the ordinary one in evo- 

 lutionary history, or the reverse ; whether the double connection may 

 be regarded as more primitive than either single one ; and how the 

 dislocation, if there be one, was brought about, are questions to which 

 we can offer no answer. 



^ Assuming, of course, that there is such a thing. 



