Data supporting the hypothesis about the epigenetic nature of deleterious mutations leading to the death of a part of the progeny are presented. It is assumed that during the life cycle "mutant" variants of formation of structural-functional loop domains occur in chromosomes that normally are corrected during meiosis. An abnormal loop changes the activity of many tens of genes, both increasing (+) and decresing (-) it, which affects the viability of homozygotes and to a lesser extent the viability of heterozygotes. Hybrids with chromosomes containing independently accumulated deleterious mutations partly restore their viability due to the complementary interactions of + and - genes.