RESTRICTED SERIES. 



79 



All these yellows seem to have been homozygous in the factors for black and for agouti. 

 Yellows of type XIX follow : 



There are two types of zygotes here, those homozygous in the agouti pattern (A) and those 

 heterozygous, the latter giving non-agouti, as well as agouti young. 



The yellows of type XVIII are also of two sorts, those homozygous in the factor for the 

 production of black (B) and those heterozygous in this factor. Thus in the following 

 table 502.21A yellow is known to be an animal homozygous in the factor B, while all 

 the other yellow animals are heterozygous, producing browns as well as blacks. 



Type XX, yellows which give browns only, are seen in the following table: 



Yelloxv X brown. 



