SEX 



107 



Fig. 24. 



Diagram showing the nature 'of the offspring from a 

 Brown Leghorn hen and an Fj cock bred from 

 Sillty hen X Brown Leghorn cock, or vice versa. 



On the other hand, the Brown Leghorn cock is 

 on our hypothesis ffppll. All his gametes conse- 

 quently contain the inhibitor factor, and when he 

 is mated with an 

 Fi hen all the 

 zygotes produced 

 must contain /. 

 None of his off- 

 spring, therefore, 

 can be fully pig- 

 mented, for this 

 condition only 

 occurs in the ab- 

 sence of the in- 

 hibitor factor among zygotes which are either homo- 

 zygous or heterozygous for P. 



The interpretation of this case turns upon the 

 constitution of the Brown Leghorn hen, upon her 

 heterozygous condition with regard to the two factors 

 /^and /, and upon the repulsion that occurs between 

 them when the gametes are formed. Through an 

 independent set of experiments this view of the 

 nature of the Brown Leghorn hen has been con- 

 firmed in an interesting way. There are fowls which 

 possess • neither the factor for pigment nor the in- 

 hibitory factor, which are in constitution ppii. Such 

 birds when crossed with the Silky give dark pig- 

 mented birds of both sexes in Fj, and the F^ genera- 

 tion consists of pigmented and unpigmented in the 

 ratio 3:1. Now a cock of such a strain crossed 

 with a Brown Leghorn hen should give only com- 

 pletely unpigmented birds. But if, as we have 

 supposed, the Brown Leghorn hen is producing 



