384 READINGS IN 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 Yellow-seeded “pure” Green-seeded “pure” 
Generation (Pr) plant (dominant) plant (recessive) 
| | 
| 
First Filial (hybrid) All the offspring were yellow-seeded 
Generation (F1) Self-fertilised they yielded 
| 
| | 
Second Filial (inbred) Yellows Yellows Greens 
Generation (F2) (pure type) oe type) (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. 
ZZ 
ee aie 
cpemmet 
a a a 
G 
1G 2G(W) iW W (F3) 
