164 Plant Genetics 



result is little hybrid vigor. Following down the dia- 

 gram it will be noted that 2, 3, and 4 of these hetero- 

 zygous sets show an increasing amount of hybrid vigor. 

 These are the facts which 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 explained the phenomena that Shull had previ- 

 ously discovered in reference to hybrid vigor. 



1. The fact of hybrid vigor. — Heterozygosis claims 

 that hybrids are vigorous because of the heterozygous 

 sets. 



2. The decrease in vigor after self-fertiUzation begins 

 again. — The greatest loss in vigor comes between the Fi 

 and the F^ generations. Thereafter the loss becomes 

 gradually less each generation, approaching a definite 

 Umit when no more loss in vigor occurs. Heterozygosis 

 explains this as follows: • 



AABBCCDDXaabbccdd 



AaBbCcDd 



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

 zygous, all four sets being heterozygous, and therefore it 

 is very vigorous. In later generations, as is well known, 

 more or less homozygous sets will be split off. Intro- 

 ducing homozygous sets into some individuals will reduce 

 the aggregate heterozygous condition of the whole crop 

 to something less than 100 per cent; there will there- 

 fore be a corresponding loss in vigor. Mathematics 

 will show why this loss is greatest between the F, and the 

 F2 generations. Thereafter the loss gradually approaches 



