MENDEL’S LAWS OF HEREDITY 385 
The grey hybrids are inbred; their offspring are grey and white 
in the proportion 3:1. If these whites are inbred they show them- 
selves “pure,” for they produce whites only for subsequent genera- 
tions. But when the greys are inbred they show themselves of two 
kinds, for one-third of them produce only greys, which go on produ- 
-cing greys; while the other two-thirds, apparently the same, produce 
both greys and whites. And so it goes on. 
Summary.—In his exceedingly clear exposition of Mendelism 
(1905) Mr. R. C. Punnett states the result thus: ‘Wherever there 
occurs a pair of differentiating characters of which one is dominant 
to the other, three possibilities exist: there are recessives which 
always breed true to the recessive character; there are dominants 
which breed true to the dominant character, and are therefore pure; 
and thirdly, there are dominants which may be called impure, and 
which on self-fertilisation (or in-breeding, where the sexes are separate) 
give both dominant and recessive forms in the fixed proportion of 
three of the former to one of the latter.” 
Schematic representation of Mendel’s Law.—Following Mr. 
Punnett’s suggestion, with slight modifications, we may use the sym- 
bols P,, P., P; for the parental, grandparental, and great-grandparental 
generations; F; for the first filial (hybrid) generations, F,, F;, F, 
for the subsquent inbred generations. The symbol D(R) means a 
dominant with the recessive character unexpressed, but potentially 
present; DD or RR means pure “extracted”? dominants or reces- 
sives—i.e., those pure forms which are sifted out from the inbreed- 
ing of “impure” dominants. 
? ‘i . . .  P3~great-grandparenta] generation 
? i : P:—grandparental generation 
D R. P:—parental generation 
D(R) . . . F*—first filial (hybrid) generation 
| 
iDD 2 D(R) 1RR . F—second filial (in- 
“Extracted” pure Impure dominants Pure recessives bred) generation 
dominants | 
| 
DD 1DD 2 D(R) 1RR RR_. Fs third generation 
| 
DD DD :DD 2D(R) 1RR RR RR . F+~fourth generation 
