Understanding genotype-sex interactions at the level of gene expression would not only shed light on mechanism, but may also identify “signatures” for variations that participate in sex-specific gene regulation. Such knowledge may also inform studies of physiological and disease traits by allowing variants to be categorized as more or less likely to participate in the sex-specific regulatory genome, and by identifying genes that are differentially regulated as candidates for sexually dimorphic traits. Box 1. Sexual DimorphismFollowing Darwin's (1859) observation that males and females may have the same “general habits of life” but “differ in structure, color, or ornament”95, research on sexual dimorphism progressed gradually from qualitative descriptions of conspicuous anatomical and behavioral traits in animals96 to elegant experiments probing the sex-specific neural circuitry of reproductive behavior in flies97,98 and mice99. The results of this century-and-a-half of research demonstrated that sexual dimorphism was taxonomically widespread and remarkably variable in the magnitude and form of its expression100,101. It is now quite obvious that sex-specific differences occur not only in conspicuous morphological traits (i.e. size, shape, and coloration) but also in a diverse
