Two models of maternal-fetal interactions are discussed. In the first, offspring are advantaged if they possess an allele absent in their mother. Polymorphism is maintained because rare alleles have an advantage when present in males. In the second, offspring are disadvantaged if they lack an allele present in their mother. Polymorphism is maintained because rare alleles have an advantage when present in females. Both classes of model are associated with a deficiency of homozygous genotypes. If the artificial assumption of symmetrical selection is relaxed, the second class of model (gestational drive) could account for the otherwise inexplicable absence of MHC polymorphism in some species.