92 



MENDELISM 



CHAP- 



factor for femaleness (F), for which the female birds 

 are heterozygous, but which is not present in the 

 males. Further, we make the assumptions {a) that 

 there is repulsion between F and / in the female 

 zygote (F/Iz), and (d) that the male Brown Leghorn 

 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 silver hen is always heterozygous for the silver 

 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 

 ffppIl[o] other hand, contains 



the inhibitor factor, but 



[f ] FfPPii 



gives 

 gfametes 



FPi\ 

 fPi / 



gives 

 gametes 



(fpI 

 Ifpl 



FfPpIi 



ffPpIi 



Fig. 22. 



not the pigmentation 

 factor. In crossing a 

 Silky hen with a Brown 

 Leghorn cock we are 

 mating two birds of the 

 constitution FfPPii 

 and ffppII^ and all the 



Scheme to illustrate the nature of the F^ „ u* J 



generation from the Silky hen and Brown l* biruS arC COnSC- 

 Leghorn cock (cf. Fig. 19). ^ , , 



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 



