W chromosome, the sex chromosomes of certain insects, birds, and fish. Females of such animals are heterogametic and have one W and one Z chromosome, whereas males are homogametic and have two Z chromosomes. The ZZ-ZW nomenclature was chosen to differentiate this system from the XX-XY system, which occurs in humans and various other animals and in which the female is homogametic and the male is heterogametic.
