344 



EVOLUTION, GENETICS, AND EUGENICS 



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 Px, Pa, P3 for the parental, grandparental, and great-grandparental 

 generations; Fi for the first filial (hybrid) generations, Fj, Fj. 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. 



D R 



I I 

 D R 



I I 

 D R 



\/ 



D(R) 



PJ — great-grandparental generation 

 P* — grandparental generation 

 P» — parental generation 

 P — first filial (hybrid) generation 



I DD 2 D(R) I RR . F»— second filial Cin- 



" Extracted" pure Impure dominants Pure recessives bred) generation 



dominants I 



DD iDD 



2D(R) 



I RR RR . FJ— third generation 



DD DD I DD 2 D(R) i RR RR RR 



F* — fourth generation 



