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X-linked dominant disorders

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X-linked dominant disorders are characterised by:

  • expression in both sexes, but with a greater incidence in females due to the greater number of X chromosomes
  • the female may be homozygous or heterozygous for the affected gene - this can only be elucidated from the family pedigree - while the male can only be heterozygous
  • the pedigree mirroring that of autosomal dominance. The only difference is that a positive father will give the condition to all of his daughters, but not his sons, whereas a positive female will transmit the trait to half of her sons and half of her daughters
  • affected males having a uniform severity of disorder, while females are affected to different degrees

Presently, there only a few known human X-linked dominant traits. With the exception of the Xg blood group, all are rare. Examples are:

  • Xg blood group
  • vitamin D resistant rickets
  • Rett's syndrome
  • Fragile X syndrome

Pseudohypoparathyroidism represents one of the difficulties in determining linkage; the apparent lack of transmission from male to male is now thought to be secondary to male hypofertility, and the disease has been reclassified as autosomal dominant.


General Practice Notebook
General Practice Notebook
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