The chromosomes which determine the gender of the organism (for organisms whose gender is determined by genes rather than by environment). In humans, fruit flies, and other animals where the male is the heterogametic sex, they are the X chromosomes and Y chromosomes. In birds, moths, and other animals where the female is the heterogametic sex, they are the W chromosomes and Z chromosomes. For humans, the sex chromosomes are the 23rd pair of all chromosomes. The sex chromosomes of most plants (when they exist) are also known as X and Y chromosomes; Ginkgo trees have a different system of sex chromosomes. Compare autosome.