The heterogametic sex (or digametic sex) is the sex of a species where an individual's gametes have non-matching sex chromosomes. In humans, the heterogametic sex is the male sex, where each gamete's sex chromosomes are X and Y. This is in contrast to the female sex, where each gamete's sex chromosomes are X and X. This arrangement is understood within the XY sex-determination system.^[1]

Non-human animals often have different sex arrangements than humans. In birds, the male sex is the homogametic sex, having two Z chromosomes, while the female sex is the heterogametic sex, with one Z and one W chromosome. In platypuses, the male sex is heterogametic while the female sex is the homogametic sex. Among the insects, Lepidopterans (butterflies and moths) have heterogametic females, but in Drosophila, males are the heterogametic sex. This arrangement is known as the ZW sex-determination system.^[2]

Heterogametic sex determination systems typically have an approximately equal sex ratio, with distributions of sexes conforming to binomial variance.^[3] However, in practice, some populations deviate from this expectation, in a phenomenon termed sex ratio distortion. Sex ratio distortion has been observed in mice,^[4] mosquitos,^[5] and other organisms^[6] and can be caused by meiotic drive elements.

Heterogamesis can lead to reduced or absent meiotic recombination between the sex chromosomes, and in some species, this extends to the autosomes, a phenomenon called achiasmy. For example, most lineages of male Drosophila melanogaster flies are achiasmic, lacking recombination on all chromosomes, although females show recombination.^[7]