The X chromosome is one of the two sex chromosomes in humans, with the other being the Y chromosome. Typically, biologically female individuals have two X chromosomes (XX) while those who are biologically male have one X and one Y chromosome (XY) . However, there are exceptions to this rule. For example, a person with five X chromosomes and one Y chromosome (XXXXXY) would be male, while an individual with only a single X chromosome and no second X or Y (i.e., XO) develops as a female and begins making ovaries, although the ovarian follicles cannot be maintained.

The X chromosome is about three times larger than the Y chromosome, containing about 900 genes, while the Y chromosome has about 55 genes. Female mammals have two X chromosomes in every cell, but one of the X chromosomes is inactivated to prevent a potentially toxic double dose of X-linked genes. This inactivated X chromosome gets condensed into a small, dense structure in the nucleus, called a Barr body, which is commonly used to determine sex.

Traits controlled by genes found only on the X chromosome are said to be sex-linked, and recessive sex-linked traits, such as hemophilia and red-green color blindness, occur far more frequently in men than in women.

In summary, the X chromosome is one of the two sex chromosomes in humans, and typically, biologically female individuals have two X chromosomes (XX) while those who are biologically male have one X and one Y chromosome (XY). The X chromosome is larger than the Y chromosome and contains about 900 genes, while the Y chromosome has about 55 genes. Traits controlled by genes found only on the X chromosome are said to be sex-linked, and recessive sex-linked traits occur more frequently in men than in women.