No. 629] SHORTER ARTICLES AND CORRESPONDENCE 559 



J (1) 



Yybb (carrying brown only). Two were selected for breeding 

 to determine in which genetic class each belonged. If both mice 

 were YyBb we should expect 



2 YyBB 1 yvBB 1 yybb 



2 Yybb 



(2) 4 YybB 2 yyBb 



4 yellow 1 black 1 brown 



The actual figures from this mating were (616 X 766) 14 

 yellow, 4 brown. This result resembles most closely that to be 

 expected if both parents were Yybb. If such were the case, all 

 of the yellows should carry brown only, never black. To test 

 them, two of the 14 yellows w^ere mated with each other 

 (2160 X 2162) . They produced 18 yellow, 1 black and 10 brown 

 young. To account for the black young, we must suppose one 

 or both parents (2160 and 2162) to have been heterozygous for 

 black and hence that one of the yellow grandparents (616 or 

 766) carried })]aek. Black young should have resulted from 

 their mating witli each other l)nt failed to do so. Black young 

 were also deficient in the mating of their descendants, 2160 and 

 2162. The ratio observed among the progeny of 2160 and 2162 

 indicates that one of them carried black rather than that both 

 of them did, for the expectation if both parents carried black, as 

 in (2), fits even more poorly than the expectation if only one 

 parent carried black, as in (4\ TIk^ doficioiicy of bla(^k young 

 in the mating of 2162 and 21 (iO is sliuwii hy tli.- ihT.-eiitairc of 

 various young observed and cxp.^ tcl : 



Per Cent, expected. 



