X chromosome
the sex chromosome that is responsible for determining femaleness in humans and other mammals. The body cells of normal females possess two X chromosomes (XX), whereas males have one X chromosome and one Y chromosome (XY). In humans, various authorities estimate that the X chromosome carries between 1,000 and 2,000 genes, including many responsible for hereditary diseases (see sex-linked). Abnormal numbers of X chromosomes lead to genetic imbalance and a range of disorders and syndromes. See also fragile X syndrome.