Figure 1.

Basic illustration of (partial) sex‐specific dominance reversal (SSDR), generating (partial) phenotypic optimization in both sexes. Given two possible alleles (a and A) at a focal large‐effect locus, the aa homozygote has a lower phenotypic value than the AA homozygote in both sexes (sexes shown in black and gray). The phenotypic value of the Aa heterozygote is closer to that of the AA homozygote in the black sex and to that of the aa homozygote in the gray sex, constituting SSDR. If high and low phenotypic values lead to higher fitness in the black and gray sexes respectively (asterisks, implying sexually antagonistic selection), then there will be net heterozygote advantage across the population, generating balancing selection that can help maintain genetic variation. The dashed line highlights the expected phenotypic value of the Aa heterozygote given purely additive allelic effects. The illustrated scenario shows symmetrical SSDR with no phenotypic sexual dimorphism in either homozygote. However, more generally, the two sexes could show different degrees of partial or complete dominance with some degree of sexual dimorphism in the homozygotes. Sex‐specific dominance, but not SSDR, would arise if phenotypic values for the Aa heterozygotes are above (or below) the additive expectation in both sexes, but to different degrees. SSDR is typically defined on the phenotypic scale (as depicted). By analogy, genome‐wide rather than single‐locus SSDRs could arise if heterozygous offspring of crosses between (relatively) homozygous parental lines show mean phenotypes that resemble different parental lines in the two sexes.