the genetic nonequivalence of the gametes of one of the sexes (male or female) in animals and dioecious plants. The so-called heterogametic sex has two kinds of gametes whose sex chromosomes differ. The form and dimensions of such gametes are usually the same. Heterogamety is based on the existence in the heterogametic sex of a pair of genetically and morphologically dissimilar sex chromosomes (heterochromosomes, heteromorphous chromosomes), which enter different gametes during meiosis.

Heterogamety is the part of the chromosomal mechanism that determines the sex of the offspring. In humans and other mammals, in some reptiles, amphibians, and fish, and also in the majority of invertebrate animals, heterogamety is characteristic of the male sex. Half of all spermatozoa contain an X chromosome, the other half a Y chromosome. The egg cells in these organisms always contain an X chromosome. (In these organisms, the female is homogametic.) In orthopterous and hemipterous insects male heterogamety consists of the fact that half the spermatozoa contain no sex chromosomes at all, and the other half contain X chromosomes. In birds, some reptiles, amphibians, and fish, as well as in butterflies heterogamety is characteristic of the female sex: one half the egg cells contain Z chromosomes, and the other half contain W chromosomes. In the overwhelming majority of dioecious plants investigated, heterogamety is characteristic of the male sex.