X chromosome

 ( Chromosomes ) 

that are due to in genes on the X chromosome are described as X linked. If the X chromosome has a genetic disease gene, it always causes illness in male patients, since men have only one X chromosome and therefore only one copy of each gene. Females, instead, require both X chromosomes to have the illness, and as a result could potentially only be a carrier of genetic illness, since their second X chromosome overrides the first. For example, hemophilia A and hemophilia B and congenital red–green color blindness run in families this way.

The X chromosome carries hundreds of genes but few, if any, of these have anything to do directly with sex determination. Early in development in females, one of the two X chromosomes is permanently inactivated in nearly all somatic cells (cells other than ovum and spermatozoon cells). This phenomenon is called X-inactivation or Lyonization, and creates a Barr body. If X-inactivation in the somatic cell meant a complete de-functionalizing of one of the X-chromosomes, it would ensure that females, like males, had only one functional copy of the X chromosome in each somatic cell. This was previously assumed to be the case. However, recent research suggests that the Barr body may be more biologically active than was previously supposed.

The partial X-inactivation is due to repressive heterochromatin that compacts the DNA and prevents the expression of most genes. Heterochromatin compaction is regulated by Polycomb Repressive Complex 2 (PRC2).

The X chromosome is notably larger and has a more active euchromatin region than its Y chromosome counterpart. Further comparison of the X and Y reveal regions of homology between the two. However, the corresponding region in the Y appears far shorter and lacks regions that are conserved in the X throughout primate species, implying a genetic degeneration for Y in that region. Because males have only one X chromosome, they are more likely to have an X chromosome-related disease.

It is estimated that about 10% of the genes encoded by the X chromosome are associated with a family of "CT" genes, so named because they encode for markers found in both tumor cells (in cancer patients) as well as in the human testis (in healthy patients).

• Klinefelter syndrome is caused by the presence of one or more extra copies of the X chromosome in a male's cells.
• Males with Klinefelter syndrome typically have one extra copy of the X chromosome in each cell, for a total of two X chromosomes and one Y chromosome (47,XXY). It is less common for affected males to 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 tall stature, learning and reading disabilities, and other medical problems. Each extra X chromosome lowers the child's IQ by about 15 points, which means that the average IQ in Klinefelter syndrome is in general in the normal range, although below average. When additional X and/or Y chromosomes are present in 48,XXXY, 48,XXYY, or 49,XXXXY, developmental delays and cognitive difficulties can be more severe and mild intellectual disability may be present.
• 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.

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. The average IQ of females with this syndrome is 90, while the average IQ of unaffected siblings is 100. Their stature on average is taller than normal females. They are fertile and their children do not inherit the condition.
• Females with more than one extra copy of the X chromosome (48, tetrasomy X or 49, pentasomy X) have been identified, but these conditions are rare.

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 features of the condition, including short stature and infertility.
• 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).

X-linked endothelial corneal dystrophy is an extremely rare disease of cornea associated with Xq25 region. Lisch epithelial corneal dystrophy is associated with Xp22.3.

Megalocornea is associated with Xq21.3-q22

Adrenoleukodystrophy, a rare and fatal disorder that is carried by the mother on the x-cell. It affects only boys between the ages of 5 and 10 and destroys the protective cell surrounding the nerves, myelin, in the brain. The female carrier hardly shows any symptoms because females have a copy of the x-cell. This disorder causes a once healthy boy to lose all abilities to walk, talk, see, hear, and even swallow. Within 2 years after diagnosis, most boys with Adrenoleukodystrophy die.

• List of X-STR markers
• Sex linkage
• X-inactivation
• Pseudoautosomal region
• Y chromosome

• Earlier versions of this article contain material from the National Library of Medicine, a part of the National Institutes of Health (USA), which, as a US government publication, is in the public domain.