IX SEX 87 



with a heterozygous dominant, the hen. Conse- 

 quently nothing but silvers appear in spite of the 

 fact that the hens are invariably heterozygous. 

 But if the gametes of the hen are always of the 

 two types Fs and/5, if she always transmits female- 

 ness and gold to her daughters, maleness and silver 

 to her sons, we should expect this to be revealed at 

 once when she is mated with a gold cock. Expecta- 

 tion is borne out by experience. Any silver hen, 

 no matter how bred, when mated to a gold cock, 

 produces only silver cocks and gold hens. She 

 transmits her colour to her sons and her sex to 

 her daughters. The results of this cross, together 

 with the nature of the F^ generation produced by 

 such Fj birds, are shown graphically on Plate V. 



On our hypothesis the sperms of the F^ silver 

 cock are of two sorts, silver and gold. Again the 

 eggs of the gold F^ hen are of two sorts, male- 

 bearing and female-bearing. Four kinds of birds 

 should therefore result in F2, viz. silver cocks and 

 silver hens, gold cocks and gold hens. These four 

 kinds should be produced in equal numbers, and 

 the experience of the breeding-pen has fully ratified 

 the demands of the hypothesis. 



An essential feature of such cases as these lies 

 in the different results obtained from reciprocal 

 crosses between two strains breeding true to type. 

 The Sebright bantams give an entirely different 

 result according as the dominant silver factor is 

 brought into the cross by the cock or by the hen, 

 and wherever there is this marked difference in 

 the reciprocal crosses the explanation will often 

 prove to be the simple one outlined above. Several 



