X chromosome

The X chromosome is one of the two sex chromosomes in humans, the other being the Y chromosome.

Human male karyotype

23 pairs of genes

The sex chromosomes form one of the 23 pairs of human chromosomes in each cell. The X chromosome spans about 155 million DNA building blocks (base pairs) and represents approximately 5 percent of the total DNA in cells.

Female sex determination

Each person normally has one pair of sex chromosomes in each cell. Females have two X chromosomes, while males have one X and one Y chromosome. Early in embryonic development in females, one of the two X chromosomes is randomly and permanently inactivated in cells other than egg cells. This phenomenon is called X-inactivation or lyonization. X-inactivation ensures that females, like males, have one functional copy of the X chromosome in each body cell. Because X-inactivation is random, in normal females the X chromosome inherited from the mother is active in some cells, and the X chromosome inherited from the father is active in other cells.

X chromosome ideogram

Psuedoautosomal regions

Some genes on the X chromosome escape X-inactivation. Many of these genes are located at the ends of each arm of the X chromosome in areas known as the pseudoautosomal regions. Although many genes are unique to the X chromosome, genes in the pseudoautosomal regions are present on both sex chromosomes. As a result, men and women each have two functional copies of these genes. Many genes in the pseudoautosomal regions are essential for normal development.

Proteins in X chromosome

The X chromosome likely contains 800 to 900 genes that provide instructions for making proteins. These proteins perform a variety of different roles in the body.

Health Conditions Related to Chromosomal Changes

The following chromosomal conditions are associated with changes in the structure or number of copies of X chromosome.

Mice X and Y chromosome

• 46,XX testicular disorder of sex development
• 48,XXXY syndrome
• 48,XXYY syndrome
• 49,XXXXY syndrome
• Intestinal pseudo-obstruction
• Klinefelter syndrome
• Microphthalmia with linear skin defects syndrome
• Triple X syndrome
• Turner syndrome
• X-linked acrogigantism
• Other chromosomal conditions