The X chromosome is one of two sex chromosomes and is present in nearly all animals that undergo sexual reproduction. Depending on the organism, it may occur alone, in two copies, or paired with a Y chromosome. The sex of an individual is directly related to the presence or absence of specific genes located on these chromosomes.

In humans, at least one X chromosome is required for life to exist. It is usually paired with either another X in a female or a Y in a male. The X chromosome is much larger than the Y, and, with the exception of a small region at one end of the X and Y chromosomes, the pseudoautosomal region, the genes present on the two chromosomes are very different. Genes located uniquely on the X chromosome are said to be X-linked.

Females have two X chromosomes and, thus, two...