Several genes in conserved clusters are expressed from only the maternal or
the paternal allele. The other allele has been genetically silenced
(‘imprinted’) by its passage through one sex. Many known imprinted
genes have effects on embryonic or trophoblast growth or fetal development,
and mutation or loss of the single active copy causes diseases such as
Prader–Willi, Angelmann and Beckwith–Wiederman syndromes.
Imprinted genes show an unusual mode of inheritance, since mutant genes have
an effect on the phenotype only if they come from the parent from which they
are expressed. This may explain some conditions which appear to be heritable
but show an inconsistant pattern in affected families. Of particular interest
is pre-eclampsia/eclampsia, the most serious complication of pregnancy,
which has some features suggesting that it results from fetal expression of
the mutant gene, but others which imply it results from maternal expression.
This could be resolved by proposing that the condition is due to mutation in a
paternally imprinted, maternally active gene which must be expressed by the
fetus in order to establish a normal placenta in the first pregnancy.
Maternal expression of the histone demethylase Kdm4a is crucial for pre-implantation development
