5 o MEXDELISM chap. 



the co-operation of the other. And we may further em- 

 phasise the fact that although the two factors thus inter- 

 act upon one another they are nevertheless transmitted 

 quite independently and in accordance with the ordinary 

 Mendelian scheme. 



One of the earliest sets of experiments demonstrating 

 the interaction of separate factors was that made by the 



French zoologist Cuenot on 

 the coat colours of mice. 



Agouti X Albino 



. r~~ 3 IT" 1 j It was shown that in cer- 



Agouti X Agouti 



tain cases agouti, which 

 is the colour of the 

 ordinary wild grey mouse, 

 behaves as a dominant to 



Agouti Black Albino 



(9) (3) (4) 



the albino variety, i.e. the F 2 generation from such a cross 

 consists of agoutis and albinos in the ratio 3:1. But in 

 other cases the cross between albino and agouti gave a 

 different result. In the Fj generation appeared only 

 agoutis as before, but the F 2 generation consisted of three 

 distinct types, viz. agoutis, albinos, and blacks. Whence 

 the sudden appearance of the new type ? The answer is a 

 simple one. The albino parent was really a black. But 

 it lacked the factor without which the colour is unable to 

 develop, and consequently it remained an albino. If we 

 denote this factor by C, then the constitution of an albino 

 must be cc, while that of a coloured animal may be CC or 

 Cc, according as to whether it breeds true to colour or can 



