X-linked dominant disorders

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.