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The X chromosome is one of the two sex-determining chromosomes in many animal species, including mammals (the other is the Y chromosome). It is a part of the XY sex-determination system and X0 sex-determination system. The X chromosome was named for its unique properties by early researchers, and this resulted in its counterpart being named the Y chromosome for the next letter in the alphabet when it was discovered later.
The sex chromosomes X X are one of the 23 homologous pairs of chromosomes in a female. The X chromosome spans more than 153 million base pairs (the building material of DNA) and represents about 5% of the total DNA in women's cells, 2.5% in men's.
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. Both males and females retain one of their mother's X chromosomes, and females retain their second X chromosome from their father. Since the father retains his X chromosome from his mother, a human female has one X chromosome from her paternal grandmother, and one X chromosome from her mother.
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 contains about 2000 genes compared to the Y chromosome containing 78 genes, out of 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 a couple thousand genes but few, if any, of these have anything to do directly with sex determination. Early in embryonic development in females, one of the two X chromosomes is randomly and permanently inactivated in nearly all somatic cells (cells other than egg and sperm cells). This phenomenon is called X-inactivation or Lyonization, and creates a Barr body. X-inactivation ensures that females, like males, have one functional copy of the X chromosome in each body cell. It was previously assumed that only one copy is actively used. However, recent research suggests that the Barr body may be more biologically active than was previously supposed.
The X-chromosome is a remarkably gene-poor region. It is composed primarily of repeated segments of DNA which do not code for proteins or any known function. Only 1.7% of the chromosome encodes for any functional proteins at all--lowest density of genes to date--and the genes themselves are very short compared to the length of the average human gene. 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). These CT genes found on the X-chromosome are estimated to account for about 90% of all the CT genes encoded within the human genome. Due to their relative abundance, it is thus hypothesized that these genes (and thus the X-chromosome) confer evolutionary fitness to human males. 
It is theorized by Ross et al 2005 and Ohno 1967 that the X-chromosome is at least partially derived from the autosomal (non-sex-related) genome of other mammals evidenced from interspecies genomic sequence alignments.
The X-chromosome is notably larger butt hole 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 which 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.
Role in diseases
- Klinefelter's 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's 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's 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. The average IQ of females with this syndrome is 90, while the average IQ of their normal siblings is 100 . Their stature on average is taller than for 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,XXXX syndrome or 49,XXXXX syndrome) have been identified, but these conditions are rare. The extra genetic material may lead to downs 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.
- 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).
XX male syndrome is a rare disorder, where the SRY region of the Y chromosome has recombined to be located on one of the X chromosomes. As a result, the XX combination after fertilization has the same effect as a XY combination, resulting in a male. However, the other genes of the X chromosome cause feminization as well.
- Sex linkage
- List of DXS markers
- 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.
- Angier, Natalie For Motherly X Chromosome, Gender Is Only the Beginning. New York Times. URL accessed on 2007-05-01.
- Macmillan Science Library (2001). Genetics on X Chromosome.
- Richard Harris (2003). Scientists Decipher Y Chromosome.
- Carrel L, Willard H (2005). X-inactivation profile reveals extensive variability in X-linked gene expression in females. Nature 434 (7031): 400–4.
- Ross M et al. (2005). The DNA sequence of the human X chromosome. Nature 434 (7031): 325–37.
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