48 



MENDELISM 



CHAP. 



agouti, black, and albino should appear in F^ in the 

 ratio 9:3:4; experiment has shown that these are 

 the only classes that appear, and that the proportions 

 in which they are produced accord closely with the 



theoretical ex- 

 pectation. Put 

 briefly, then, the 

 explanation of 

 this case is that 

 all the animals 

 are black, and that 

 we are dealing 

 with the presence 

 and absence of 

 two factors, a 

 colour developer 

 {C), and a colour 

 modifier {G), both 

 acting, as it were, 



Diagram to illustrate the nature of the F.2 generation 



which may arise from the mating of agouti with Upon a SUbstratUm 

 albino in mice or rabbits. /- 1 1 1 rr^^ t^ 



of black. The F„ 

 generation really consists of the four classes agoutis, 

 blacks, albino agoutis, and albino blacks in the ratio 

 9:3:3:1. But since in the absence of the colour 

 developer C the colour modifier G can produce no 

 visible result, the last two classes of the ratio are 

 indistinguishable, and our F^ generation comes to 

 consist of three classes in the ratio 9:3:4, instead 

 of four classes in the ratio 9:3:3:1. 



This explanation was further tested by experi- 

 ments with the albinos. In an F^ family of this 

 nature there ought to be three kinds, viz. albinos 

 homozygous for G (ccGGBB), albinos heterozygous 



Fig. 8. 



