X chromosome

The X chromosome is one of the two sex chromosomes in humans and other animals (the other is the Y chromosome). It is a part of the XY sex-determination system.

Function

The sex chromosomes are one of the 23 pairs of human chromosomes. The X chromosome spans more than 153 million base pairs (the building material of DNA) and represents about 5 percent of the total DNA in cells.

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 somatic cells (cells other than egg and sperm 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.

Identifying 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 between 900 and 1,200 genes.

Genes on chromosome X are among the estimated 20,000 to 25,000 total genes in the human genome. Genetic disorders that are due to mutations in genes on the X chromosome are described as X linked.

The X chromosome carries hundreds of genes but few, if any, of these have anything to do directly with sex determination.

Women have two X chromosomes, and hence two copies of each gene. One of the two chromosomes appears to be inactivated (Lyonization) into a Barr body, and it was previously assumed that only one copy is actively used. However, recent research (Carrel & Willard 2005) suggests that the Barr body may be more biologically active that was previously supposed.

Role in disease

Numerical abnormalities

Klinefelter syndrome:

• Klinefelter syndrome is caused by the presence of one or more extra copies of the X chromosome in a male's cells. Extra genetic material from the X chromosome interferes with male sexual development, preventing the testicles from functioning normally and reducing the levels of testosterone.
• Typically, males with Klinefelter syndrome have one extra copy of the X chromosome in each cell, for a total of two X chromosomes and one Y chromosome (47,XXY). Less commonly, affected males may have two or three extra X chromosomes (48,XXXY or 49,XXXXY) or extra copies of both the X and Y chromosomes (48,XXYY) in each cell. The extra genetic material may lead to mental retardation and other medical problems.
• Klinefelter syndrome can also result from an extra X chromosome in only some of the body's cells. These cases are called mosaic 46,XY/47,XXY.

Triple X syndrome (also called 47,XXX or trisomy X):

• This syndrome results from an extra copy of the X chromosome in each of a female's cells. Females with trisomy X have three X chromosomes, for a total of 47 chromosomes per cell. Researchers are not yet certain why an extra copy of the X chromosome is associated with tall stature and learning problems in some girls and women.
• Females with more than one extra copy of the X chromosome (48,XXXX or 49,XXXXX) have been identified, but these conditions are rare. The extra genetic material may lead to mental retardation and other medical problems.

Turner syndrome:

• This results when each of a female's cells has one normal X chromosome and the other sex chromosome is missing or altered. The missing genetic material affects development and causes the characteristic features of the condition, including short stature and infertility (the inability to conceive a child).
• About half of individuals with Turner syndrome have monosomy X (45,X), which means each cell in a woman's body has only one copy of the X chromosome instead of the usual two copies. Turner syndrome can also occur if one of the sex chromosomes is partially missing or rearranged rather than completely missing. Some women with Turner syndrome have a chromosomal change in only some of their cells. These cases are called Turner syndrome mosaics (45,X/46,XX).

Chromosomal conditions involving the sex chromosomes often affect sex determination (whether a person has the sexual characteristics of a male or a female), sexual development, and the ability to have children (fertility). The signs and symptoms of these conditions vary widely and may range from mild to severe. They can be caused by missing or extra copies of the sex chromosomes or by structural changes in the chromosomes.

Other conditions

Many diseases are due to mutations on the X chromosome. These are characteristic as they occur much more often in men than in women. Moreover, they inherit in a characteristic manner: fathers do not transmit them to their sons, but their daughters are carriers as they inherit their father's X chromosome, and their sons have a 50% chance of inheriting that X chromosome. Diseases well known for their X-linked recessive inheritance are hemophilia (types A and B), and color blindness. There are few examples of X-linked dominant diseases; the best known in this category is vitamin D resistant rickets . The following genetic disorders are associated with the X chromosome:

References

• Earlier versions of this article contain material from the National Library of Medicine (http://www.nlm.nih.gov/copyright.html) , a part of the National Institutes of Health (USA,) which, as a US government publication, is in the public domain.
• Carrel L, Willard HF. X-inactivation profile reveals extensive variability in X-linked gene expression in females. Nature 2005;434:400-4. DOI.