1 62 Outline of Genetics 



that are in the heterozygous condition in the Fi genera- 

 tion. Thus in the first case shown in the diagram there 

 is only a single heterozygous set {Dd), and the result is 

 little hybrid vigor. Following down the diagram it 

 will be noted that 2,3, and 4 of these heterozygous sets 

 bring an increasing amount of hybrid vigor. These are 

 the facts that lie at the basis of East's theory which he 

 calls heterozygosis. This term should not be confused 

 with heterosis, which is commonly used to express merely 

 the fact of hybrid vigor. 



We shall now consider how this conception of hetero- 

 zygosis serves to account for the phenomena that Shull 

 had previously discovered in connection with hybrid 

 vigor. 



1. The fact of hybrid vigor. — Heterozygosis suggests 

 that hybrids are vigorous on account of the heterozygous 

 sets of factors that they contain. 



2. The decrease in vigor after self-fertilization begins 

 again.^ — The greatest loss in vigor comes between the Fj 

 and F2 generations. Thereafter the loss becomes gradu- 

 ally less each generation, approaching a definite limit 

 beyond which no further loss in vigor occurs. Heterozy- 

 gosis explains this as follows: 



AABBCCDDXaabhccdd = AaBhCcDd. 



In this case the Fi generation is 100 per cent heterozy- 

 gous, all four factor sets being heterozygous, and there- 

 fore it is very vigorous. In later generations, as is well 

 known, more or less homozygous sets will be split off. 

 Introducing homozygous sets into some individuals will 

 reduce the aggregate heterozygous condition of the whole 



