MENDEL'S LAWS OF HEREDITY 385 



The grey hybrids are inbred; then: 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 a , P, for the parental, grandparental, and great-grandparental 

 generations; F x for the first filial (hybrid) generations, F,, F,. F 4 

 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. 



D R . . . P great-grandparental generation 

 D R . . . P 1 grandparental generation 

 D R . . . P 1 parental generation 



D(R) 



P first filial (hybrid) generation 



.Jr 



)D 2D(R) iRR . F-*econd filial (in- 



1 Extracted " pure Impure dominants Pure recessives bred) generation 



dominants | 



DD iDD 2D(R) iRR RR . F third generation 



RR RR . 



)D DD i DD 2 D(R) i RR RR RR . F* fourth generation 



