Hybrid Vigor 163 



population to something less than too per cent; there 

 will therefore be a corresponding loss in \ig(jr. 



If the genotype of the F^ population be considered (a simpler 

 example, AABBXaabb, will suffice), some very clear conclusions 

 may be drawn. The F2 population is heterogeneous with respect 

 to hybrid vigor, in sharp contrast with the F,, where all the individ- 

 uals showed the same amount of hybrid vigor. In the F^ there 

 will be one genotype which is heterozygous with respect to all of 

 the factor pairs involved (as was the F,), and which, therefore, 

 shows the maximum amount of vigor. There will be other geno- 

 types which are homozygous with respect to all the factor pairs, 

 and show no vigor. And there will be still other genotypes which 

 are parti}'' heterozygous and partly homozygous, and show an 

 intermediate amount of vigor. This heterogeneity of the F, gen- 

 eration with respect to amount of hybrid vigor is in agreement 

 with the actual experimental results. 



If the average vigor of the whole F^ population be computed, 

 in terms of relative numbers of factor sets in the heterozygous 

 condition, this will be found to have a value of 50 per cent, in con- 

 trast with the o per cent of the original grandparental generation 

 and the 100 per cent of the Fj. On the same basis the F3 will be 

 found to have 25 per cent, the F4 12.5 per cent, and so on, exactly 

 one-half of the vigor being lost with each succeeding generation of 

 inbreeding. This serves to account for Shull's observation that 

 the greatest loss in vigor is between the Fi and F, generations. 

 Thereafter the loss gradually approaches the limit when the per- 

 fectly homozygous condition is reached for the whole population, 

 and then there can be no more loss in vigor. 



3. A cross between sister and brother elTects nothing. 

 — This is evident, for it introduces no heterozygosity. 



4. "A cross between ])lants belonging to two self- 

 fertilized families results in a i)ri)geny of as great \igor, 

 size, and productiveness as are possessed by families 

 that have never been self-fertilized." Heterozygosis 

 accounts for this by showing that a cross between two 



