Genetics 



305 



this interaction of several factors, different combinations may result. 

 For instance, in fowl, each variety has a typical type of comb. The 

 inheritance of this comb character is controlled by two pairs of allelo- 

 morphs. The Wyandotte breed has a low regular type comb known as 

 "rose" ; Brahmas have a narrow three-ridged comb known as "pea" ; 

 while Leghorns and some others have a single comb consisting of a 

 simple upright blade. The breeding of a rose-combed fowl with a pea- 

 combed one produces a fowl with a comb entirely different from either 

 parent, a "walnut comb," so named due to its resemblance to a walnut 

 meat. These results can only be explained by the fact that the type of 

 comb is controlled by two pairs of allelomorphs, each on a separate 

 chromosome, one designated as R, the others as P. If at least one R is 

 present without a F, a rose comb results ; if but one P is present without 

 the R, a pea comb results. If both R and P are present, a walnut 

 comb results ; if neither is present, a single comb results. 



PARENTS 



f; 



Fig. 94. — A dihybrid cross in which two pairs of allelomorphs interact to control 

 a single phenotypic character, in this case the comb appearance in fowl. 



Complementary factors demonstrate another type of cross in which 

 the interaction of genes is shown. An excellent example of this inter- 

 action is found in the coat color of guinea pigs. This is controlled by 

 two sets of complementary factors. One of these, designated C, allows 

 the development of color; the other, A, causes banding of the black 

 hairs with yellow, resulting in the agouti pattern. If C is present with 

 only a, the animal is black ; if A alone is present with c, or if only a and 

 c are present, the animal is an albino. In the F., the ratio is 9 agoutis 

 to 3 blacks and 4 albinos. 



