Evolution of mating preferences and major histocompatibility complex genes

House mice prefer mates genetically dissimilar at the major histocompatibility complex (MHC). The highly polymorphic MHC genes control immunological self/nonself recognition; therefore, this mating preference may function to provide "good genes" for an individual's offspring. However, the evidence for MHC-dependent mating preferences is controversial, and its function remains unclear. Here we provide a critical review of the studies on MHCdependent mating preferences in mice, sheep, and humans and the possible functions of this behavior. There are three adaptive hypotheses for MHC-dependent mating preferences. First, MHCdisassortative mating preferences produce MHC-heterozygous offspring that may have enhanced immunocompetence. Although this hypothesis is not supported by tests of single parasites, MHC heterozygotes may be resistant to multiple parasites.