X-linked recessive inheritance

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X-linked recessive inheritance

X-linked recessive inheritance is a mode of inheritance in which a mutation in a gene on the X chromosome causes the phenotype to be expressed (1) in males (who are necessarily hemizygous for the gene mutation because they have only one X chromosome) and (2) in females who are homozygous for the gene mutation (i.e., they have a copy of the gene mutation on each of their two X chromosomes).

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 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.[1]

Some scholars have suggested discontinuing the terms dominant and recessive when referring to X-linked inheritance due to the multiple mechanisms that can result in the expression of X-linked traits in females, which include cell autonomous expression, skewed X-inactivation, clonal expansion, and somatic mosaicism.[2]

Sex Differences In Phenotype/Genotypes and Frequency[edit]

In humans, generally "men are affected and women are carriers" for two reasons. The first is the simple statistical fact that if the X-chromosomes is a population that carry a particular X-linked mutation at a frequency of 'f' (for example, 1%) then that will be the frequency that men are likely to express the mutation (since they have only one X), while women will express it at a frequency of f2 (for example 1% * 1% = 0.01%) since sup they have two X's and hence two chances to get the normal allele. Thus, X-linked mutations tend to be rare in women. The second reason for female rarity is that women who express the mutation must have two X chromosomes that carry the trait and they necessarily got one from their father, who would have also expressed the trait because he only had one X chromosome in the first place. If the trait lowers the probability of fathering a child or induces the father to only have children with women who aren't carriers (so as not to create daughters who are carriers rather than expressers and then only if no genetic screening is used) then women become even less likely to express the trait.

Examples[edit]

Most common[edit]

The most common X-linked recessive disorders are:[3]

Less common disorders[edit]

Theoretically, a mutation in any of the genes in the Category:Genes on chromosome X may cause disease, but below are some notable ones, with short description of symptoms:

See also[edit]

External links[edit]

References[edit]

  1. ^ OMIM X-linked Genes
  2. ^ http://www.ncbi.nlm.nih.gov/pubmed/15316978
  3. ^ GP Notebook - X-linked recessive disorders Retrieved on 5 Mars, 2009
  4. ^ OMIM Color Blindness, Deutan Series; CBD
  5. ^ Michael Price (8 October 2009). "Case Closed: Famous Royals Suffered From Hemophilia". ScienceNOW Daily News. AAAS. Retrieved 9 October 2009. 
  6. ^ Evgeny I. Rogaev et al. (8 October 2009). "Genotype Analysis Identifies the Cause of the "Royal Disease"". Science. Retrieved 9 October 2009. 
  7. ^ "Hemophilia B". National Hemophilia Foundation.
  8. ^ Carlo Gelmetti; Caputo, Ruggero (2002). Pediatric Dermatology and Dermatopathology: A Concise Atlas. T&F STM. p. 160. ISBN 1-84184-120-X. 
  9. ^ "X-linked Agammaglobulinemia: Immunodeficiency Disorders: Merck Manual Professional". Retrieved 2008-03-01. 
  10. ^ XLA information by St. Jude Children's Hospital
  11. ^ "Favism". Patient.co.uk. Accessed December 5, 2012.