52 



MENDELISM 



CHAP. 



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 devel- 

 oper (C), and a colour 

 modifier (G), both act- 

 ing, as it were, upon 

 a substratum of black. 

 The F2 generation 

 really consists of the 

 four classes agoutis, 

 blacks, albino agoutis, 

 and albino blacks in 

 the ratio 9:3:3: i. 

 But since in the 

 absence of the colour 



which may arise from the mating of agouti with deVClopCr C the Colour 

 albino in mice or rabbits. 



modifier G can, pro- 

 duce no visible result, the last two classes of the ratio are 

 indistinguishable, and our Fj 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 experiments 

 with the albinos. In an. Fa family of this nature there 

 ought to be three kinds, viz. albinos homozygous for G 

 (GGccBB), albinos heterozygous for G (GgccBB), and 

 albinos without G (ggccBB). These albinos are, as it 

 were, like photographic plates exposed but undeveloped. 



Fig. 8. 

 Diagram to illustrate the nature of the Fa generation 



