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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]

Sex Differences In Phenotype/Genotypes and Frequency[edit | edit source]

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 they have two X's and hence two chances to get the normal allele. Thus, X-linked mutations tend to be rare in women.

Examples[edit | edit source]

Examples of interest tom psychologists include:

Most common[edit | edit source]

The most common X-linked recessive disorders are:[2]

  • Color blindness; a classic example of an X-linked trait because it is easy to explain the phenotype and it's relatively common [3] 7% to 10% of men are red-green colorblind, making the above calculations predict 0.49% to 1% of women would be affected. Its commonness may be attributable to its not being a serious disability in most cases and an actual advantage in some situations (for example, not being distracted by some of the color in color based camouflage).It is also known as daltonism.
  • Hemophilia A; another famous example because it was found in European royal families who intermarried and were famous enough that their pedigrees could be established and offered in textbooks as a "famous example" of an X-linked trait that had been documented in history books before mendelian genetics was understood.
  • Duchenne muscular dystrophy; muscular dystrophy associated with mutations in the dystrophin gene, characterized by rapid progression of muscle degeneration, eventually leading to loss in ambulation, respiratory failure and death.
  • Becker's muscular dystrophy; milder form of Duchenne, causes slowly progressive muscle weakness of the legs and pelvis.
  • Hemophilia B; a blood clotting disorder caused by a mutation of the Factor IX gene, leading to a deficiency of Factor IX. It is rarer than haemophilia A. It's also called Christmas disease[citation needed]

Less common disorders[edit | edit source]

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 | edit source]

External links[edit | edit source]

References[edit | edit source]

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