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X-linked recessive disorders
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Some features of X-linked recessive inheritance are:

  • only males are affected
  • there is no variation in expression, the disorder always follows a typical course
  • heterozygous females are clinically unaffected but carry the mutant gene
  • only rarely will a female manifest the signs of an X-linked disease; this is usually due to atypical lyonization, a new mutation in the other X chromosome, a carrier with Turner's syndrome, or X-autosome translocation
  • milder signs of X-linked disorders may evolve in the female due to normal lyonization

Approximately 290 X-linked recessive conditions are known, the following are the most frequently encountered:

  • red-green colour blindness
  • non-specific X-linked mental retardation
  • Duchenne muscular dystrophy
  • Becker muscular dystrophy
  • Haemophilia A (factor VIII)
  • Haemophilia B (factor IX)
  • X-linked ichythyosis
  • X-linked agammaglobulinaemia

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