VI] MENDELIAN HEREDITY 73 



production of pigment in general (P) and the other 

 for the determination of the actual colour of that 

 pigment (6^ = grey, J5 = 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 Pg from the cross wild grey 

 X albino then contain G derived from their wild 

 grandparent. These mated with black give grep 

 offspring because grey is dominant^ over black, and 

 the black individual introduces the factor P which 

 was absent in the albino. These grey rats (generation 

 Fs 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^), 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. 



