88 MENDELISM chap. 



other cases of the nature have ah-eady been worked 

 out in poultry. The barred plumage of the Plymouth 

 Rock is such a one. The barred character is 

 dominant to full black, but a barred hen is always 

 heterozygous for the barred factor. Mated with a 

 black cock she always gives barred cocks and black 

 hens. The cock in a pure breeding strain of Ply- 

 mouth Rocks is homozygous for the barring factor. 

 As a matter of fact, most strains will occasionally 

 throw a few blacks, and in such cases the blacks are 

 always pullets. This is because most strains are not 

 pure bred in the sense that every cock is homozygous 

 for the barring factor. Occasional cocks are hetero- 

 zygous. When such cocks are mated with barred 

 hens they produce barred and blacks in the ratio 

 3:1, just as the F^ silvers shown on Plate V. produce 

 silvers and golds in the same ratio. And just as 

 all the golds in the latter case are hens, so also 

 are all the black hens in the former. The breeder 

 of Plymouth Rocks who wishes to ensure that his 

 strain should throw no blacks has only to make 

 sure that the cocks from which he breeds are homo- 

 zygous. This he can easily do by making a test 

 rttating with a black hen. If the cock throws no , 

 blacks in a dozen or so birds he is almost sure to be 

 homozygous, and can subsequently be mated with 

 barred hens with the certainty that the strain will 

 thenceforth throw no blacks. For the occasional 

 blacks depend upon the nature of the cock, the 

 barred hens being always of the same constitution, 

 no matter how they have been bred. 



It is perhaps natural to regard the barred 

 character of the plumage in the Plymouth Rock 



