vi] HEREDITY 73 



production of pigment in general (P) and the other 

 for the determination of the actual colour of that 

 pigment (6r = grey, B = black). Neither G nor B 

 can produce any visible effect in the absence of P ; 

 a rat without P (represented by p) is thus an albino. 

 The extracted albinos in F 2 from the cross wild grey 

 x albino then contain G derived from their wild 

 grandparent. These mated with black give grey 

 offspring because grey is dominant 1 over black, and 

 the black individual introduces the factor P which 

 was absent in the albino. These grey rats (generation 

 F 3 in the diagram) are thus heterozygous in the pair 

 of factors grey and black (G and B) and in the factors 

 presence and absence of P (P and p). They will thus 

 produce gametes GP, Gp, BP, Bp, which in meeting 

 at random will give 9 zygotes containing G and P, 3 

 containing B and P, 3 containing G and p, 1 con- 

 taining B and p. But the combinations Gp and Bp, 

 not having P, are albinos, and so we get 9 greys, 

 3 blacks, 4 whites. 



If in the example just given nothing were known 

 of the origin of the white rat which was crossed with 

 the black (in the generation marked F 2 ), it would be 

 said that a white variety crossed with a black had 



1 This explanation has been simplified by the omission of the fact 

 that G and B do not represent factors for separate pigments, but that 

 G consists in the addition of a pigment to hairs already containing B. 

 A character dominant in this way is called ' epistatic,' see below p. 75. 



