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 inconsistent 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.