46 MENDELISM chap. 



of coloured and white birds in the ratio 9 : 7, and 

 when the experiment was made this was actually 

 shown to be the case. With the growth of know- 

 ledge it is probable that further striking parallels 

 of this nature between the plant and animal worlds 

 will be met with. 



Before quitting the subject of these experiments, 

 attention may be drawn to the fact that the 9 : 7 

 ratio is in reality a 9:3:3:1 ratio in which the 

 last three terms are indistinguishable owing to the 

 special circumstances that neither factor can produce 

 a visible effect without the co-operation of the other. 

 And we may further emphasise the fact that although 

 the two factors thus interact upon one another they 

 are nevertheless transmitted quite independently 

 and in accordance with the ordinary Mendelian 

 scheme. 



One of the earliest sets of experiments demon- 

 strating the interaction of separate factors was that 



made by the French 

 Agouti X Albino zoologist Cu^not on the 



I 1 1 coat colours'of mice. It 



Agouti X Agouti ^^g shown that in cer- 



tain cases agouti, which 



» r~ „,' , r^;„„ is the colour of the 



Agouti Black Albino 



(9) (3) (4) ordinary wild grey 



mouse, behaves as a 

 dominant to the albino variety, i.e. the F^ 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 F 

 generation appeared only agoutis as before, but the 

 Fg generation consisted of three distinct types, viz. 



