72 



HEREDITY 



[CH. 



factors, in the absence of either of which no colour 

 is produced. An actual example will make this 

 clearer. A white rat is mated with a wild (brown 

 or 'grey') rat, and since colour dominates over its 

 absence the F l heterozygotes are all grey, like wild 

 rats. These grey heterozygqtes mated together give 



grey x white 



/\ 



grey x grey O 



3 grey 



1 white 



GGpp 



black 



BBPP 



grey 



GBPp 



grey 



.GBPp 



9 grey 



1G6PP 

 2GB PP 

 2GG Pp 

 4GBPp 



3 black 



1 BBPP 

 2BBPp 



4- white 



1GG pp 

 2GB pp 

 1BB pjj 



coloured and albino in the ratio of three to one. If 

 now one of these extracted albinos is mated with 

 a black rat, the offspring may not be black but grey, 

 and such grey individuals paired together will give 

 young in the ratio of 9 grey, 3 black, 4 white. 



The explanation is as follows. For the production 

 of colour, two factors must be present, one for the 



