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X chromosome

The X chromosome is one of the two sex 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.

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 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.

Diseases well known for their X-linked recessive inheritance are hemophilia (types A and B), and colour 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:

  • Alport syndrome
  • Androgen insensitivity syndrome
  • Charcot-Marie-Tooth disease
  • Coffin-Lowry syndrome
  • Fabry disease
  • Fragile X syndrome
  • Glucose-6-phosphate dehydrogenase deficiency
  • Hemophilia
  • Incontinentia pigmenti
  • Lesch-Nyhan syndrome
  • Menkes disease
  • Duchenne and Becker’s muscular dystrophy
  • Nonsyndromic deafness and X-linked nonsyndromic deafness
  • Ornithine transcarbamylase deficiency
  • Rett syndrome
  • Spinal and bulbar muscular atrophy
  • X-linked severe combined immunodeficiency
  • X-linked sideroblastic anemia

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.