The X chromosome is one of the two sex chromosomes in mammals (the other is the Y chromosome). It is a part of the XY sex-determination system. Figure 1: Chromosome. ... The human Y chromosome is one of two sex chromosomes, it contains the genes that cause testis development, thus determining maleness. ... The XY sex-determination system is a well-known 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% 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. Embryos (and one tadpole) of the wrinkled frog (Rana rugosa). ... A human ovum An ovum (loosely, egg or egg cell) is a female sex cell or gamete. ... The signifier sperm can refer to: (mass noun, from Greek sperma = seed) a substance which consists of spermatozoa and which is a component of semen (mass noun) semen itself (informally, count noun with plural sperm or sperms) a single spermatozoon (= sperm cell) sperma ceti (Latin ceti, genitive of cetus = whale...

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. The Lyon hypothesis states that in cells with multiple X chromosomes, all but one is inactivated during mammalian embryogenesis. ... In those species in which sex is determined by the presence of the Y or W chromosome rather than the diploidy of the X or Z, a Barr body is the inactive X chromosome in a female cell, or the inactive Z in a male (Lyon, 2003), rendered inactive in...

Role in disease

Numerical abnormalities

Klinefelter syndrome: XXY karyotype Klinefelters syndrome is a condition caused by a chromosome abnormality in males (specifically, a nondisjunction); sufferers have a pair of X sex chromosomes instead of just one. ...

• 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): Testosterone is a steroid hormone from the androgen group. ... Mental retardation (also called mental handicap and, as defined by the UK Mental Health Act 1983, mental impairment and severe mental impairment) is a term for a pattern of persistently slow learning of basic motor and language skills (milestones) during childhood, and a significantly below-normal global intellectual capacity as... This article contains information that has not been verified and thus might not be reliable. ...

• 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). There is also evidence that Turner syndrome may be linked to incresed levels of aggression and violent behaviour.
• 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 caused by mutations on the X chromosome. These diseases occur far more frequently in males because they only have one X chromosome, so even if the gene is recessive, they will have the disease. Females must receive a copy of the gene from both parents to have such a disease. However, they will still be carriers if they receive one copy of the gene. Recessive genes on the X chromosome that cause serious diseases are usually passed from female carriers to their sons and daughters. This is because males, who always have the disease and are not just carriers, would have to father a daughter to pass on the gene. This is unlikely because severe genetic diseases often cause death in childhood or early adulthood. Even those males who survive childhood are unlikely to father children because a sickly male will be less likely to find a mate. However, if the disease shows up late in life, or is not severe, he will pass the gene to all of his daughters. He cannot pass it to his sons because a male receives his X chromosome from his mother. A mother with one copy of the gene has a 50% chance of passing it to her children of both sexes, but her daughters will just be carriers of the gene unless their father has it too. In biology, mutations are changes to the genetic material (usually DNA or RNA). ...

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: Haemophilia or hemophilia is the name of any of several hereditary genetic illnesses that impair the bodys ability to control bleeding. ... Color blindness in humans is the inability to perceive differences between some or all colors that other people can distinguish. ... The word dominant has several possible meanings: In music theory, the dominant or dominant note (second most important) of a key is that which is a perfect fifth above the tonic; in just intonation the note whose pitch is 1. ...

Alport syndrome is a hereditary disease characterized by hematuria. ... Androgen insensitivity syndrome (AIS) is a set of disorders of sexual differentiation that results from mutations of the gene encoding the androgen receptor. ... The foot of a person with Charcot-Marie-Tooth. ... Coffin-Lowry syndrome is a condition associated with mental retardation and delayed development, characteristic facial features, and skeletal abnormalities. ... Fabrys disease (also known as Anderson-Fabry Disease, Angiokeratoma Corporis Diffusum and Hereditary Dystopic Lipidosis) is a lysosomal storage disease resulting from an X-linked mutation. ... Fragile X Syndrome is the most common inherited cause of mental retardation, and is associated with autism. ... Glucose-6-phosphate dehydrogenase (G6PD) deficiency is an X-linked recessive hereditary disease featuring nonimmune hemolytic anemia in response to a number of causes. ... Haemophilia or hemophilia is the name of any of several hereditary genetic illnesses that impair the bodys ability to control bleeding. ... Incontinentia pigmenti (IP) is a disorder that affects the skin, hair, teeth, and nails. ... Lesch-Nyhan syndrome, or LNS, is a serious genetic disorder that affects mostly males. ... Menkes disease, also called the kinky hair disease or Menkes kinky hair syndrome, is a disease of abnormal copper metabolism. ... Duchenne muscular dystrophy (DMD) (also known as pseudohypertrophic muscular dystrophy or muscular dystrophy - Duchenne type) is an inherited disorder characterized by rapidly progressive muscle weakness which starts in the legs and pelvis and later affects the whole body. ... Beckers muscular dystrophy (also known as Benign pseudohypertrophic muscular dystrophy) is an X-linked recessive inherited disorder characterized by slowly progressive muscle weakness of the legs and pelvis. ... Nonsyndromic deafness is hearing loss that is not associated with other signs and symptoms. ... Ornithine transcarbamylase deficiency (OTC), the most common of the Urea Cycle Disorders, is a rare metabolic disorder, occurring in one out of every 40000 births. ... Rett syndrome (Rett syndrome, Retts disorder) is a progressive neurological disorder. ... It has been suggested that this article or section be merged into Severe combined immunodeficiency. ... The porphyrias are inherited or acquired disorders of certain enzymes in the heme biosynthetic pathway (also called porphyrin pathway). ...

See also

Sex linkage Sex linkage is the phenotypic expression of an allele that is dependent on the sex of the individual and is directly tied to the sex chromosomes. ...

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.