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 1 heterozygotes are all grey, like wild 

 rats. These grey heterozygotes mated together give 



grey X white 



*5 



grey x grey 



3 grey 



1 white x black 



GGpp ^ ^ BB PP 



grey 



GB Pp 



grey 



GBPp 



9 grey 



1 GGPP 



2 GB PP 

 2GG Pp 

 4 GB Pp 



3 black 



1 BB PP 



2BBPji 



4 white 



7 GG pp 



2 GB pp 

 1 BB pp 



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 



