GENEHCS 



673 



There is, in addition, a P gene which produces purple-colored kernels 

 if both C and R genes are present. The P gene is supplementary to the 

 other two pairs of genes and C and R are complementary. 



The coat color of Duroc-Jersey pigs represents a slightly different 

 type of gene interaction. Two independent pairs of genes (R-r and S-s) 

 regulate coat color; at least one dominant of each pair must be present 

 to give the full, red-colored coat. Partial color, sandy, results when only 

 one type of dominant is present and an animal which is homozygous for 

 both recessives (rrss) has a white-colored coat. The mating of two 

 different strains of sandy-colored pigs, RRss X "SS, yields offspring all of 

 which are red, and the mating of two of these red F^ individuals pro- 

 duces an F2 generation in the ratio of 9 red : 6 sandy : 1 white (Fig. 33.2). 



Rose coml) 

 RRpp 



Pea. comb 

 rrPP 



Fi 



All walnut comb 

 RrPp 



RP 



rP 



rp 



RP Rp 



rP 



rp 



9 walnut 



3 rose 



3 pea. 





i single. 

 Figure 33.3. Diagram of the inheritance of comb types in chickens. See text for 



discussion. 



