TOO W. GORDON WHALEY 



range of environmental adaptability. It is equally true that certain hybrids 

 exhibit vigor within only relatively narrow environmental limits. For lack of 

 evidence it must be assumed that the distinction lies in the differences be- 

 tween the patterns of hybridity and in the action of the genes responsible for 

 the hybrid advantages. 



Any attempt to explain the genetic basis of heterosis must make initial 

 recognition of one fact. The phenomenon can involve only the recombination 

 of alleles already existing in the population or populations from which the 

 hybrid organisms have been developed; unless, by rare chance, mutation 

 should take place just prior to or just after the actual crossing. We are thus 

 concerned with an interpretation limited to different types of recombina- 

 tions, and to different kinds of gene action resulting from these recombina- 

 tions. 



GENETIC MECHANISM OF HETEROSIS 



Consideration of the characteristics of dominance and heterozygosity has 

 been of primary importance to investigators concerned with interpretation 

 of the genetic mechanism of heterosis. Jones's dominance of linked factors 

 hypothesis (1917) probably is still the most popular explanation of the 

 genetic basis of heterosis. 



Dobzhansky (1941) and his co-workers, and many others, have recorded 

 that in most species there has been, in the course of evolution, accumulation 

 of deleterious recessive characters, which when homozygous reduce the 

 efficiency of the organism — but which in the heterozygous condition are 

 without efficiency-reducing effects. This revelation calls for a reshaping of no- 

 tions regarding the nature of the favorable effects of the dominant alleles, but 

 does not otherwise modify the structure of the explanation. The favorable- 

 ness of the action of many of the dominant alleles probably is not the result 

 either of directional mutation producing more favorable dominants or of 

 selection tending to eliminate the unfavorable dominants. Instead, it may 

 be due to the accumulation in populations of deleterious recessive mutations. 

 These, if their effects are not too deleterious, often can be piled up in sig- 

 nificant numbers. 



The piling-up of such deleterious recessives is probably one of the reasons 

 why heterosis is a much more important phenomenon in such a plant as corn 

 than it is, for example, in the tomato. Corn has been handled for hundreds 

 or even thousands of years in a manner that has made possible the accumula- 

 tion in populations of relatively large numbers of deleterious recessive modi- 

 fiers. The tomato is more than 90 per cent self-pollinated, and any great 

 accumulation of deleterious modifiers is unlikely. Corn populations char- 

 acteristically contain thousands of individuals, and wind pollination makes 

 for maintenance of heterozygosity. In tomato, the effective breeding popula- 

 tion size approaches one, and deleterious mutations would tend to become 



