a chromosome that determines whether an individual is female or male. 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 usually is considered to be a female if the body cells contain the XX combination of chromosomes and male if the cells contain the XY combination, regardless of physical traits or signs of hermaphroditism. Diseases coded by genes that are carried only on a sex chromosome (usually the X chromosome) are called sex-linked conditions.