342 EVOLUTION, GENETICS, AND EUGENICS 



D9XR^orR$XD4 . . . Parent-forms (P') 



\/ 



D(R) Hybrid-offspring (P) 



3.D 



I R . Generation of inbred hybrids (F») 



iD 



+ 



2D(R) 



SP 



iR R 



I D + 2 D(R) 

 I 



D D 3D 



iR R R 



iD+2D(R) 



I 

 D D D 



R R R 



(F4) 



(Fs) 



carry with them the possibility of having offspring with the recessive 

 character; that is to say, the recessive character remains latent in 

 the inheritance. 



When these D(R)s are inbred (self-fertilised, in the case of peas) 

 they have offspring (F^), some of which resemble the recessive parent, 

 while others resemble the dominant parent, and these occur in the 

 proportion of 1:3. When those resembling the recessive parent are 

 inbred, they breed true — i.e., they give rise to a line of pure recessives. 

 Those resembUng the dominant parent are all apparently alike, but 

 their subsequent history shows that they may be divided into a set 

 which breed true to the dominant type and a set which behave like the 

 first generation of hybrids — -i.e., they go on splitting up into dominant- 

 like forms and pure recessives. These two sets occur in the propor- 

 tions of 1:2. 



A case of peas. — Let us consider a concrete case. Peas with 

 rounded seeds were crossed with peas having angular wrinkled seeds. 

 In the offspring the character of roundness was dominant ; the angular 

 wrinkled character had disappeared or receded. It was not lost, as 

 the next generation showed. 



The hybrid offspring, aU with rounded seeds, were allowed to self- 

 fertilise. In their progeny roundish seeds and angular wrinkled seeds 

 occurred in the proportions of 3:1. Here were the recessives again, 

 and when they were allowed to self-fertilise they produced pure reces- 

 sives only, with angular wrinkled seeds. 



