X chromosome

The X chromosome is one of the two sex chromosomes in humans, and it is the larger and gene-richer of the pair. Where the Y chromosome is small and carries relatively few genes, the X holds hundreds of genes, many of which have nothing to do with sex and are simply needed for ordinary body functions.

Typical females carry two X chromosomes, while typical males carry one X and one Y. Because males have only a single copy of the X, any gene on it shows up directly with no second copy to mask it. This is why several conditions caused by genes on the X — such as red-green color blindness, hemophilia and Duchenne muscular dystrophy — are seen more often in males, who lack a backup copy of those genes.

Having two X chromosomes in females would otherwise mean a double dose of X-linked genes compared with males. Cells balance this through X-chromosome inactivation, in which one X in each female cell is largely switched off and condensed into a compact structure called a Barr body. The choice of which X is silenced is essentially random from cell to cell, so females are mosaics of two cell populations.