94 



MENDELISM 



CHAr. 



these two series of gametes is given in Fig. 24. 

 Of the eight different kinds of zygote formed only 

 one contains P in the absence of /, and this is a 

 female. The result, as we have already seen, is in 

 accordance with the experimental facts. 



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 



Fig. 24. , '^ 



Diagram showing the nature of the offspring from a CUIlUlLlOn Oniy 

 Brown Leghorn hen and an Fi cock bred from /-jrnirQ in fVif* aK 

 Silky hen X Brown Leghorn cock, or z/?V^ z/fTT^a. UCCUIij 111 LIIC dU- 



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 F^, and the F^ genera- 



