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 {Ffli), and {U) 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 

 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 



[f ] FfP.Pii ffppn[c5] other hand, contains 



gfmTtes gametes the inhibitor factor, but 



Ppj\ |-£ J not the pigmentation 



fPi / f \fpl factor. In crossing a 



I Silky hen with a Brown 



Leghorn cock we are 



"fa 



I — I 



Ff Ppli ffPpIi mating two birds of the 



[?] [(^] constitution FfPPii 



^"'- ^^- and ffppll, and all the 



Scheme to illustrate the nature of the Fj _-, i • j 



generation from the Silky hen and Brown r birds are COHSC- 

 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 {ffPPii) with a Brown Leghorn 

 hen which on our assumption is heterozygous for the 



