IOO 



MENDELISM 



is homozygous for the inhibitor factor (/), but that 

 the hen Brown Leghorn is always heterozygous for 

 this factor just in the same way as the female of the 

 currant moth is always heterozygous for the grossu- 

 lariata factor. We may now proceed to show how 

 this explanation fits the experimental facts which 

 we have given. 



The Silky is pure for the pigmentation factor, 

 but does not contain the inhibitor factor. The 



Brown Leghorn, on the 

 ffppllfo] other hand, contains 

 the inhibitor factor, but 

 not the pigmentation 

 factor. In crossing a 

 Silky hen with a Brown 

 Leghorn cock we are 

 mating two birds of the 

 constitution FfPPii 

 and ffppll, and all the 



[f ] FfPPii 



FIG. 22. 



Scheme to illustrate the nature of the Fj 

 generation from the Silky hen and Brown 

 Leghorn cock (cf. Fig. 23). 



F I birds are conse- 



quently heterozygous 

 for both P and /. In such birds the pigment is 

 almost but not completely suppressed, and as both 

 sexes are of the same constitution with regard to 

 these two factors they are both of similar appearance. 

 In the reciprocal cross, on the other hand, we are 

 mating a Silky male (ffPPit) with a Brown Leghorn 

 hen which on our assumption is heterozygous for the 

 inhibitor factor (/), and in constitution therefore is 

 Ffppli. Owing to the repulsion between F and / 

 the gametes produced by such a bird are Fpi and fpl 

 in equal numbers. All the gametes produced by 

 the Silky cock are fPi. Hence the constitution of 



