The occurrence of a ring 18, an accessory bisatellited fragment, and trisomy 21 within one sibship.

A family is presented in which there were three different chromosome abnormalities in the children although the parents were cytogenetically normal. The proband had the karyotype 46,XX, r(18) (p11q23), a phenotypically normal brother was 47,XY,+mar, and another brother was a typical case of Down's syndrome, karyotype 47,XY,+21. There is nothing in the parents' history that provides grounds for a hypothesis to explain the coincidence of the abnormalities.