Anencephaly and spina bifida: an etiologic hypothesis.

The families of subjects with anencephaly and spina bifida show an increased incidence of the two deformities which appears to be confined to matrilineal relatives. Although the recurrence risk among sibs is 4-5%, affected twins are usually discordant for the deformities. These facts suggest that the two malformations are caused by cytoplasmic abnormalities, and possibly by mutations of cytoplasmic genes.