236 



EVOLUTION, GENETICS, AND EUGENICS 



The dominants, however, were not all pure dominants, for when 

 they were allowed to self-fertilise they produced one-third pure domi- 

 nants and two-thirds "impure" dominants, the latter being distin- 

 guished by the fact that in their offspring recessives reappeared in the 

 proportion of one recessive to three dominants. 



The outstanding facts, taking the case of yellow-seeded and green- 

 seeded peas, may be thus summarised: — 



Parental 



Generation (Pi) 



First Filial (hybrid) 

 Generation (Fi) 



Yellow-seeded "pure' 

 plant (dominant) 



Green-seeded "pure' 

 plant (recessive) 



All the offspring were yellow-seeded 

 Self-fertilised they yielded 



Second Filial (inbred) Yellows 

 Generation (F2) (pure type) 



Yellows 

 (impure type) 



Greens 

 (pure type) 



Third Filial (inbred) Yellows Yellows Yellows Greens Greens 



Generation (F3) (pure type) (pure) (impure) (pure) (pure type) 



Thus intercrossing of forms with contrasted characters results not 

 in transitional blends, but in the dominance of one character and the 

 recession of another. Self-fertilisation (the extreme of inbreeding) 

 of the hybrids results in a number of pure recessives and a number of 

 dominants in the proportion 1:3; some of these dominants (one- third) 

 are pure, and produce only dominants; some (two-thirds) are appar- 

 ently pure, but produce dominants and recessives in the old propor- 

 tion, 3:1. 



A case of mice. — Let us take a concrete case from among animals. 

 A grey house-mouse is crossed with a white mouse; the offspring are 

 all grey. Greyness is dominant ; albinism is recessive. 



1 G 2 G(W) 



1 W 



W (FO 



