Monosomy 18p and pure trisomy 18p in a family with translocation (7;18).

A three generation pedigree is described in which there are two carriers of translocation t(7;18). Two members of the family have trisomy 18p and a stillborn child had monosomy 18p and holoprosencephaly. Another stillborn child probably had holoprosencephaly; the karyotype was not analysed. Based on this observation, the occasional occurrence of holoprosencephaly in monosomy 18p (10% of previously reported cases) may not be the result of the expression of a recessive mutant gene in the hemizygous state, as assumed up to now.