The X chromosome is one of the two sex chromosomes in humans (the other is the Y chromosome). The sex chromosomes form one of the 23 pairs of 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.
Each person usually has one pair of sex chromosomes in each cell. Females typically have two X chromosomes, while males typically have one X and one Y chromosome. Early in the embryonic development of people with two X chromosomes, one of the 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 people with two X chromosomes have only one functional copy of the X chromosome in each cell. Because X-inactivation is random, normally, the X chromosome inherited from one parent is active in some cells, and the X chromosome inherited from the other parent is active in other cells.
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, males and females each have two functional copies of these genes. Many genes in the pseudoautosomal regions are essential for normal development.
Identifying the genes on each chromosome is an active area of genetic research. Because researchers use different approaches to predict the number of genes on each chromosome, the estimated number of genes varies. The X chromosome likely contains 900 to 1,400 genes that provide instructions for making proteins. These proteins perform a variety of different roles in the body.
Related Genetics Topics
- Klinefelter syndrome — MedlinePlus Genetics
- Turner syndrome — MedlinePlus Genetics
- Triple X syndrome — MedlinePlus Genetics
- 46,XX testicular disorder of sex development — MedlinePlus Genetics
- Microphthalmia with linear skin defects syndrome — MedlinePlus Genetics
- 48,XXYY syndrome — MedlinePlus Genetics
- Intestinal pseudo-obstruction — MedlinePlus Genetics
- X-linked acrogigantism — MedlinePlus Genetics
- 49,XXXXY syndrome — MedlinePlus Genetics
- 48,XXXY syndrome — MedlinePlus Genetics
Source: MedlinePlus Genetics, U.S. National Library of Medicine.
