X linked dominant gen

An X linked dominant gene will give rise to a disorder that
affects both hemizygous males and heterozygous females.
Although dominant, females may be less severely affected than
males, as in X linked hypophosphataemia (vitamin D-resistant
rickets) and oculomotor nystagmus, because of X inactivation
which results in expression of the mutant allele in only a
proportion of cells. The gene is transmitted through families in
the same way as X linked recessive genes: females transmit the
mutation to half their sons and half their daughters; males
transmit the mutation to all their daughters and none of their
sons. The pedigree, however, resembles autosomal dominant
inheritance except that there is no male to male transmission
and there is an excess of affected females. In some disorders the
condition appears to be lethal in affected males, for example
focal dermal hypoplasia (Goltz syndrome) and incontinentia
pigmenti. In these families there will be fewer males than
expected, half of the females will be affected and all surviving
males will be unaffected. An affected woman therefore has a
one in three chance of having an affected child and two thirds
of her children will be girls. Rett syndrome is a disorder that
affects girls almost exclusively and usually occurs sporadically,
since affected females do not reproduce. This disorder has been
shown to be due to a mutation in a gene located at Xq24,
confirming that it is an X linked dominant condition.