384 



EVOLUTION, GENETICS, AND EUGENICS 



accord with the conceptions that have been presented, and only two 

 assumptions are necessary: (i) that dominance is absent, and two 

 doses have twice the effect of one: (2) that the independent similar 

 factors are cumulative in their operation, and are paired with their 

 absence in the hybrid. This was Nilsson-Ehle's conception, and of 



Fig. 74. — Diagram illustrating Nilsson-Ehle's explanation of 15:1 ratio 

 obtained in F2 generation from cross between red-grained and white-grained 



wheat. {From Conlkr and Coulter.) 



course he tested it by further experimental work, the results consist- 

 ently confirming the conception. 



Since it is important to fix this conception clearly in mind, another 

 type of diagram may represent the facts even more clearly. The 

 proportion of individuals showing the various degrees of redness in the 

 F2 is graphically recorded in Fig. 75, each dot representing one dose 

 of the factors in question. 



