The homologous chromosomes that are dissimilar in the heterogametic sex. There are the x chromosome, the y chromosome, and the W, Z chromosomes (in animals in which the female is the heterogametic sex (the silkworm moth bombyx mori, for example)). In such cases the W chromosome is the female-determining and the male is ZZ. (From King & Stansfield, A dictionary of genetics, 4th ed)