The isogenic, highly heterotic parthenoclone 29, originating from a hybrid silkworm female, was transformed via unisexual reproduction (meiotic and ameiotic parthenogenesis) into four genotypical variants differing in well-known various levels of hetero zygosity and combinations of useful and harmful genes. A comparison of these changes with the heterosis level made it possible to discover that both heterosity for adaptively neutral genes (overdominance hypothesis) and the number of allelic pairs, each of them being heterozygous for a favourable, completely dominant gene (dominance hypothesis) play no decisive role in the intensity of heterosis. The level of heterosis is largely determined by the relationship between the effects of useful and harmful genes, the first falling into the category of semidominant, cumulatively acting genes which control viability. Their favourable, joint well-coordinated effects, unlike those of genes which control quantitative characters, increase in relation to the number of genes in a geometric rather than an arithmetic progression. The interaction between semilethal genes is subjected to the same regularity. The high combining ability of parthenoclone 29 variants is determined by the number and homozygosity of the useful genes.