APA Dictionary of Psychology

a chromosome that determines whether an individual is female, male, or intersex. Humans and other mammals have two sex chromosomes: the X chromosome, which carries genes for certain sexual traits and occurs in both females and males; and the smaller Y chromosome, which is normally found only in males. An individual is usually assigned the sex of female if the body cells contain the XX combination of chromosomes and of male if the cells contain the XY combination. Diseases coded by genes that are carried only on a sex chromosome (usually the X chromosome) are called sex-linked conditions.