A chromosome that operates in the sex-determining mechanism of a species. Many animals have two different types of sex chromosome. For example, in mammals there is a large X chromosome and a much smaller Y chromosome; the female has two matching X chromosomes, making it the homogametic sex, whereas the male has one X chromosome and one Y chromosome, making it the heterogametic sex. In other animal groups, such as butterflies and moths, birds, and reptiles, the situation is reversed: males have two matching W chromosomes, while females have one W chromosome and one Z chromosome. Sex chromosomes carry genes governing the development of sex organs and secondary sexual characteristics. They also carry other genes unrelated to sex (see sex linkage). A few plants have distinct sex chromosomes, one example being the maidenhair tree (Gingko biloba), in which male and female trees are distinguished by their X and Y chromosomes. See sex determination; testis-determining factor; X inactivation.