SEX 



in 



FIG. 24. 



f ji -p Diagram showing the nature of the offspring from a 



:OWn Brown Leghorn hen and an F l cock bred from Silky 

 hen X Brown Leghorn cock, or vice versa. 



hen all the zygotes produced must contain 7. None of his 

 offspring, therefore, can be fully pigmented, for this con- 

 dition only occurs in the absence of the inhibitor factor 

 among zygotes which 

 are either homozy- 

 gous or heterozygous 

 for P. 



The interpretation 

 of this case turns 

 upon the constitu- 

 tion 



Leghorn hen, upon 

 her heterozygous condition with regard to the two factors 

 F and 7, 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 confirmed in an interesting way. 

 There are fowls which possess neither the factor for pig- 

 ment nor the inhibitory factor, which are in constitution 

 ppii. Such birds when crossed with the Silky give dark 

 pigmented birds of both sexes in FI, and the F 2 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 completely unpigmented 

 birds. But if, as we have supposed, the Brown Leghorn 

 hen is producing gametes Fpi and fpl, the male birds 

 produced by such a cross should be heterozygous for 7, 



