SEX 



109 



gives 

 gametes 



FPi\ 

 fPi / 



gives 

 gametes 



Upl 



FfPpli 



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 FI birds are 

 consequently heterozygous for both P and 7. In such 

 birds the pigment is al- 



most but not completely " ff P p " [ <* ' 



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 

 (ffPPii) with a Brown Leghorn hen which on our as- 

 sumption is heterozygous for the inhibitor factor (7), 

 and in constitution therefore is Ffppli. Owing to 

 the repulsion between F and 7 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 FI male birds produced by 

 this cross is ffPpli 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- 



ffPpIi 



uu 



FIG. 22. 



Scheme to illustrate the nature of the Y 1 gener- 

 ation from the Silky hen and Brown Leghorn 

 cock (cf. Fig. 23). 



