

io8 MENDELISM CHAP. 



behaved alike, whether they were from the Silky hen or 

 from the Silky cock. We have, therefore, the paradox 

 that the FI hen, though herself deeply pigmented, cannot 

 transmit this condition to any of her offspring when she 

 is mated to the unpigmented Brown Leghorn, but that, 

 when similarly mated, the FI cock can transmit this pig- 

 mented condition to a quarter of his female offspring 

 though he himself is almost devoid of pigment. 



Now all these apparently complicated results, as well 

 as many others to which we have not alluded, can be ex- 

 pressed by the following simple scheme. There are three 

 factors affecting pigment, viz. (i) a pigmentation fac- 

 tor (P); (2) a factor which inhibits the production of 

 pigment (7) ; and (3) a 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 (b) that the male Brown Leghorn is ho- 

 mozygous for the inhibitor factor (7), 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 al- 

 ways heterozygous for the grossulariata factor. We may 

 now proceed to show how this explanation fits the ex- 

 perimental 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 other hand, contains the inhibitor factor, but not the 



