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''Main Article'': <u>[[Sex linkage]]</u>{{short description|Mode of inheritance}}
[[File:X-linked recessive (2).svg|thumb|right|upright=1.75|X-linked recessive inheritance]]
'''X-linked recessive inheritance''' is a mode of [[Mendelian inheritance|inheritance]] in which a [[mutation]] in a [[gene]] on the [[X chromosome]] causes the [[phenotype]] to be always expressed in males (who are necessarily [[
X-linked inheritance means that the gene causing the trait or the disorder is located on the X chromosome. Females have two X chromosomes while males have one X and one [[Y chromosome]]. Carrier females who have only one copy of the mutation do not usually express the phenotype, although differences in [[X-chromosome inactivation]] (known as [[skewed X-inactivation]]) can lead to varying degrees of clinical expression in carrier females, since some cells will express one X [[allele]] and some will express the other. The current estimate of sequenced X-linked genes is 499, and the total, including vaguely defined traits, is 983.<ref>{{cite web|url=https://www.ncbi.nlm.nih.gov/Omim/mimstats.html|title=OMIM X-linked Genes|website=nih.gov|access-date=3 May 2018|url-status=
== Patterns of inheritance ==
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* The first is that affected fathers cannot pass X-linked recessive traits to their sons because fathers give Y chromosomes to their sons. This means that males affected by an X-linked recessive disorder inherited the responsible X chromosome from their mothers.
* Second, X-linked recessive traits are more commonly expressed in males than females.<ref>{{Cite book|date=8 July 2009|title=Understanding Genetics: A New York, Mid-Atlantic Guide for Patients and Health Professionals|url=https://www.ncbi.nlm.nih.gov/books/NBK115561/|access-date=9 June 2020|website=National Center for Biotechnology Information}}</ref> This is due to the fact that males possess only a single X chromosome, and therefore require only one mutated X in order to be affected. Women possess two X chromosomes, and thus must receive two of the mutated recessive X chromosomes (one from each parent). A popular example showing this pattern of inheritance is that of the descendants of Queen Victoria and the blood disease [[Haemophilia|hemophilia]].<ref>{{Cite web|date=2014-03-04|title=History of Bleeding Disorders|url=https://www.hemophilia.org/Bleeding-Disorders/History-of-Bleeding-Disorders|access-date=2020-06-09|website=National Hemophilia Foundation|language=en}}</ref>
* The last pattern seen is that X-linked recessive traits tend to skip generations, meaning that an affected grandfather will not have an affected son, but could have an affected grandson through his daughter.<ref>{{Cite book|last=Pierce|first=Benjamin A.|title=Genetics: A Conceptual Approach|publisher=Macmillan Learning|year=2020|isbn=978-1-319-29714-5|pages=154–155}}</ref> Explained further, all daughters of an affected man will obtain his mutated X, and will then be either carriers or affected themselves depending on the mother. The resulting sons will either have a 50% chance of being affected (mother is carrier), or 100% chance (mother is affected). It is because of these percentages that we see males more commonly affected than females.{{cn|date=February 2024}}
== Pushback on recessive/dominant terminology ==
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===Most common===
The most common X-linked recessive disorders are:<ref name=gpnotebook>[http://www.gpnotebook.co.uk/simplepage.cfm?ID=-1341784030 GP Notebook - X-linked recessive disorders] {{webarchive|url=https://web.archive.org/web/20110613144555/http://www.gpnotebook.co.uk/simplepage.cfm?ID=-1341784030 |date=2011-06-13 }} Retrieved on 5 Mars, 2009</ref>
* [[Red–green color blindness]],
* [[Hemophilia A]], a [[coagulation|blood clotting]] disorder caused by a [[mutation]] of the [[Factor VIII]] [[gene]] and leading to a deficiency of Factor VIII. It was once thought to be the "royal disease" found in the descendants of Queen Victoria. This is now known to have been Hemophilia B (see below).<ref>{{cite web|author= Michael Price|title= Case Closed: Famous Royals Suffered From Hemophilia|url=
* [[Hemophilia B]], also known as Christmas disease,<ref>[http://www.hemophilia.org/NHFWeb/MainPgs/MainNHF.aspx?menuid=181&contentid=46&rptname=bleeding "Hemophilia B".] {{webarchive|url=https://web.archive.org/web/20071201092338/http://www.hemophilia.org/NHFWeb/MainPgs/MainNHF.aspx?menuid=181&contentid=46&rptname=bleeding |date=2007-12-01 }} National Hemophilia Foundation.</ref> a [[coagulation|blood clotting]] disorder caused by a [[mutation]] of the [[Factor IX]] [[gene]] and leading to a deficiency of Factor IX. It is rarer than [[hemophilia A]]. As noted above, it was [[Haemophilia in European royalty|common]] among the descendants of Queen Victoria.
* [[Duchenne muscular dystrophy]], which is associated with mutations in the [[dystrophin]] gene. It is characterized by rapid progression of muscle degeneration, eventually leading to loss of skeletal muscle control, respiratory failure, and death.
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{{see also|X-linked intellectual disability}}
Theoretically, a mutation in any of the [[:Category:Genes on human chromosome X|genes on chromosome X]] may cause disease, but below are some notable ones, with short description of symptoms:
* [[Adrenoleukodystrophy]]; leads to progressive brain damage, failure of the adrenal glands, and eventually death.
* [[Alport syndrome]]; glomerulonephritis, endstage kidney disease, and hearing loss. A minority of Alport syndrome cases are due to an autosomal recessive mutation in the gene coding for [[type IV collagen]].
* [[Androgen insensitivity syndrome]]; variable degrees of undervirilization and/or infertility in XY persons of either sex
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* [[Centronuclear myopathy]]; where cell nuclei are abnormally located in skeletal muscle cells. In CNM the nuclei are located at a position in the center of the cell, instead of their normal ___location at the periphery.
* [[Charcot–Marie–Tooth disease|Charcot–Marie–Tooth disease (CMTX2-3)]]; disorder of nerves (neuropathy) that is characterized by loss of muscle tissue and touch sensation, predominantly in the feet and legs but also in the hands and arms in the advanced stages of disease.
* [[Coffin–Lowry syndrome]]; severe intellectual disability sometimes associated with abnormalities of growth, cardiac abnormalities, and kyphoscoliosis as well as auditory and visual abnormalities.
* [[Fabry disease]]; A lysosomal storage disease causing anhidrosis, fatigue, angiokeratomas, burning extremity pain, and ocular involvement.
* [[Hunter syndrome]]; potentially causing hearing loss, thickening of the heart valves leading to a decline in cardiac function, obstructive airway disease, sleep apnea, and enlargement of the liver and spleen.
* [[Hypohidrotic ectodermal dysplasia]], presenting with hypohidrosis, hypotrichosis, and hypodontia
* [[Kabuki syndrome]] (the ''[[KDM6A]]'' variant); multiple congenital anomalies and intellectual disability.
* [[Spinal and bulbar muscular atrophy]]; muscle cramps and progressive weakness▼
* [[Lesch–Nyhan syndrome]]; neurologic dysfunction, cognitive and behavioral disturbances including self-mutilation, and uric acid overproduction (hyperuricemia)
* [[Lowe syndrome]]; hydrophthalmia, cataracts, intellectual disabilities, aminoaciduria, reduced renal ammonia production, and vitamin D-resistant rickets
* [[Menkes disease]]; sparse and coarse hair, growth failure, and deterioration of the nervous system
* [[Nasodigitoacoustic syndrome]]; misshaped nose, [[brachydactyly]] of the [[distal phalanges]], sensorineural deafness
* [[Nonsyndromic deafness]]; hearing loss
* [[Norrie disease]]; cataracts
* [[Occipital horn syndrome]]; deformations in the skeleton
* [[Ocular albinism]]; lack of pigmentation in the eye
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* [[Siderius X-linked mental retardation syndrome]]; cleft lip and palate with intellectual disability and facial dysmorphism, caused by mutations in the histone demethylase [[PHF8]]
* [[Simpson–Golabi–Behmel syndrome]]; coarse faces with protruding jaw and tongue, widened nasal bridge, and upturned nasal tip
▲* [[Spinal and bulbar muscular atrophy]] (SBMA), also known as Kennedy's disease; muscle cramps and progressive weakness
* [[Spinal muscular atrophy]] caused by [[UBE1]] gene mutation; weakness due to loss of the motor neurons of the spinal cord and brainstem
* [[Wiskott–Aldrich syndrome]]; eczema, thrombocytopenia, immune deficiency, and bloody diarrhea
* [[X-linked severe combined immunodeficiency]] (SCID); infections, usually causing death in the first years of life
* [[X-linked sideroblastic anemia]]; skin paleness, fatigue, dizziness, and enlarged spleen and liver.
==See also==
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[Female X-linked disorders]
* [https://www.
[[Category:Genetics]]
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