Functional mosaicism

Functional mosaicism occurs in all females as only one
X chromosome remains active in each cell. The process of
X inactivation occurs in early embryogenesis and is random.
Thus, alleles that differ between the two chromosomes will be
expressed in mosaic fashion. Carriers of X linked recessive
mutations normally remain asymptomatic as only a proportion of
cells have the mutant allele on the active chromosome.
Occasional females will, by chance, have the normal
X chromosome inactivated in the majority of cells and will
then manifest systemic symptoms of the disorder caused by
the mutant gene. In X linked dominant disorders such as
incontinentia pigmenti, female gene carriers have patchy skin
pigmentation that follows Blaschko’s lines because of the mixture
of normal and mutant cells in the skin during development.