NATURE AND ORIGIN OF HETEROSIS 221 



BALANCED EUHETEROSIS 



Balanced heterosis is due to the occurrence of a rather special class of 

 mutations and gene combinations, which confer on heterozygotes a higher 

 adaptive value, or a higher agricultural usefulness than is found in the cor- 

 responding homozygotes. 



The conditions most frequently found in heterozygotes are either domi- 

 nance and recessiveness, when the heterozygote is more or less similar to one 

 of the homozygotes, or phenotypical intermediacy between the homozygotes. 

 A heterozygote may, however, be in some respects phenotypical ly more ex- 

 treme than either homozygote. Thus, a heterozygote may be more viable, 

 more productive, or otherwise exceed both homozygotes in some positive or 

 negative quality. This condition is sometimes spoken of as overdominance 

 (Hull). 



Although overdominance is, by and large, an exceptional situation, it is of 

 particular interest to a student of population genetics, and especially to a 

 student of heterosis. Suppose that a certain gene is represented in a popula- 

 tion by a series of alleles, A\ A^,A^ . . . which are deleterious in homozygous 

 condition, AKi\ A-A^, A^A^ . . . , but which show a relatively higher fitness 

 in heterozygotes A^A'^, A'^A^, A~A^ . . . , etc. Natural or artificial selection 

 would preserve in the population all the variants A\ A", A^ . . . , regardless 

 of how poorly adapted the homozygotes may be. In fact, one or all homozy- 

 gotes may be semilethal or even lethal, and yet selection will establish an 

 equilibrium at which every one of the variants will be present with a definite 

 frequency. This equilibrium can easily be calculated if the selective dis- 

 advantages of the homozygotes, compared to the heterozygotes, are known. 

 The resulting situation is referred to as balanced polymorphism. 



Balanced polymorphism may be produced by mutations in single genes, 

 provided that the heterozygotes exhibit overdominance in fitness in some 

 environments. This has been demonstrated, among others, by Gustafsson 

 and Nybom. They observed several mutations in barley that were deleterious 

 in homozygotes, but produced heterozygotes superior to the ancestral "nor- 

 mal" homozygotes. Ford and others showed that certain color variants in 

 butterflies, which are inherited as though caused by a single genetic change, 

 are maintained in natural populations by the same mechanism. 



Detailed data are available on balanced polymorphism in several species 

 of Drosophila, in which natural populations are very often polymorphic for 

 gene arrangements in some chromosomes. These gene arrangements differ in 

 inversions of blocks of genes. Thus, in certain populations of Drosophila 

 pseiidoobscura from Southern California, at least 70 per cent of the wild indi- 

 viduals are inversion heterozygotes. In populations of Drosophila willisloni 

 from central Brazil (Goyaz), an average individual is heterozygous for as 

 many as nine inversions, and very few individuals are homozygous. 



