HEREDITY 



If now yellow individuals are mated with 

 each other we expect three sorts of young to 

 be produced numerically, as 1 : 2 : 1, viz. 1 Y Y, 

 2 Y B, and IBB. But since observation shows 

 that only two combinations are formed which 

 contain yellow to one not containing yellow, 

 and since further all yellows which survive 

 are found to be heterozygous (YB), it must 

 be that the expected Y Y individual either is 

 not produced or straightway perishes. As to 

 which of these two contingencies happens we 

 also have experimental evidence. Mr. Little 

 finds that yellow mice when mated to black 

 ones produce larger litters of young than when 

 they are mated to yellow ones. The average- 

 sized litter contains something like 5.5 young 

 when the mate is a black animal, but only 4.7 

 when it is a yellow animal. It is evident, then, 

 that about one young one out of a litter per- 

 ishes when both parents are yellow, and this 

 undoubtedly is the missing yellow-yellow zy- 

 gote. The yellows which are left are hetero- 

 zygous yellow-black zygotes, and they are to 

 those that perish as 2:1. They are also to 

 tlie non-yellow zygotes as 2:1, the ratio ob- 



60 



