86 MENDELISM chap. 



all of his sperms are of the constitution /S} The 

 gold hen produces two kinds of eggs. All are 

 alike in lacking the factor for silver, but half of 

 them contain F while the other half lack this factor. 

 The former are, of course, the female-producing eggs, 

 and the latter the male-producing ones. The F^ 

 cocks, therefore, may be represented in constitution 

 a.s //Ss, the F^ hens as F/Ss. The sperms produced 

 by such cocks are of two kinds, viz. /S and /s : 

 the eggs produced by the F^ hens are on hypo- 

 thesis also of two kinds, viz. Fs and /S. Each of 

 these two kinds of eggs has an equal chance of 

 being fertilised by either of the two classes of sperms. 

 Consequently, as Plate V. shows, four classes of birds 

 are produced in F^. The peculiar relation between 

 colour and sex is explained on our hypothesis 

 by the fact that all of her male -producing eggs 

 contain the silver factor, while none of her female- 

 producing eggs contain it. Since all males must 

 receive S from their mother, the F, generation of 

 this sex consists of homozygous and heterozygous 

 silvers in equal numbers : since the female-producing 

 eggs alone lack the factor for silver, all of the re- 

 cessive golds in the F^ generation must be hens. 



If the view just set out is a true one, it follows 

 that no silver hen can be homozygous for 5. In 

 strains of silvers which breed true the hens are 

 always heterozygous for 5. With regard to the 

 factor for silver the type of mating in such a strain 

 is always that of a homozygous dominant, the cock, 



^ On Plate V. the male-bearing eggs and sperms are marked 

 by a capital M instead of by a small /as in the account given 

 in the text. 



