SEX 



109 



pigmentation factor. In crossing a Silky hen with a 

 Brown Leghorn cock we are mating two birds of the 

 constitution FJPPii and ffppll, and all the Fi birds are 

 consequently heterozygous for both P and /. In such 

 birds the pigment is al- 

 most but not completely ' 

 suppressed, and as both 

 sexes are of the same 

 constitution with regard 

 to these two factors they 

 are both of similar ap- 

 pearance. 



In the reciprocal cross, 

 on the other hand, we 

 are mating a Silky male 

 (f/PPii) with a Brown Leghorn hen which on our as- 

 sumption 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 the F x male birds produced by 

 this cross is fJPpli as before, but the female birds must 

 be all of the constitution FfPpii. The Silky cock trans- 

 mits the fully pigmented condition to his daughters, 

 because the gametes of the Brown Leghorn hen which con- 

 tain the factor for femaleness do not contain the inhibi- 



gives 

 gametes 



gives 

 gametes 



FPi) (fpl 

 fPi / X I fpl 



1 

 FfPpIi 



[?] 



ffPpii 



Fig. 



22. 



Scheme to illustrate the nature of the F x gener- 

 ation from the Silky hen and Brown Leghorn 

 cock (cf. Fig. 23 J. 



