On the inheritance of intersexuality in swine

Data of Breeuwsma (1970) were analyzed in an attempt to discriminate between major gene vs. multifactorial modes of inheritance of intersexuality in swine. Of 3708 females, 160 were intersexes with external phenotypes ranging from normal female (normal overlap) to testicular pseudohermaphrodite. Environment (litter size, parity, hormone treatment of dam) influenced detection of carriers but not origin of intersexes. Normal overlaps lowered penetrance, partly due to deaths in competition with male littermates. Phenocopies (intersex with unusual genotype or with karyotype other than 38,XX) were rare. Sex ratio variation between mating types could be ascribed to the ascertainment method. Segregation ratio estimates for female sibships increased from those with at least one to those with at least two intersexes less than expected for polygenic inheritance. The latter could not be ruled out (heritability of liability by three methods was 78%), but duplicate epistasis provided a more parsimonious explanation. Separation of litters from retrospectively known carriers into identifying and post-identifying groups produced patterns of segregation estimates supporting inheritance by few rather tha many genes. Crossbred intersexes indicate homology of genes for intersexuality in several European breeds of pigs.