ESTIMATES OF GENETIC VARIANCES AND LEVEL OF DOMINANCE IN MAIZE

Abstract

HE dominant favorable gene hypothesis, first proposed by BRUCE (1910), T attributes heterosis to accumulated effects of loci at which the more favorable allele contributed by one parent is manifest rather than the unfavorable allele contributed by the other parent. The concepts of linkage (JONES 191 7) and large number of loci (COLLINS 1921; SINGLETON 1941) make the hypothesis compatible with the observation of symmetrical F, distributions and the lack of SUperior homozygous inbred lines. The concept of heterozygote superiority was first put into the framework of Mendelian inheritance by EAST (1936) with the suggestion that if alleles had diverged slightly in function, a heterozygote might have an advantage over the homozygotes. HULL (1945) introduced the term "overdominance" to denote het-erozygote superiority over either homozygote at the locus level, and proposed this as the most important genetic effect in corn populations. The two hypotheses are not mutually exclusive; i.e., the heterosis observed in crossbred corn populations might be not only due to loci with partial to complete dominance but also due to overdominant loci. Furthermore, the effects of repulsion phase linkage of favorable dominant genes tend to mimic overdominant effects , and are indistinguishable from the effects of overdominance when the frequency of recombination is near zero. Pseudo-overdominant effects, in contrast to true overdominant effects, tend to diminish with the approach to linkage equilibrium under random mating at a rate proportional to the recombination frequency between loci. Therefore, not only the relative importance of dominance versus overdominance, but also the rate of decrease of pseudo-overdominant effects with random mating, has important implications regarding the evolutionary significance of heterosis and its practical utilization in plant breeding. Several approaches to this problem have been pursued giving somewhat conflicting conclusions. CROW (1948) considered the theoretical relationship between heterosis and the effect of deleterious recessives maintained in the population by mutation, and concluded that observed heterosis was not entirely attributable to the dominance of favorable genes. BRIECER (1950) presents an argument in favor