Molecular studies of trisomy 18

The authors have determined the parental origin of 50 cases of trisomy 18. In 48 cases the additional chromosome was maternal in origin, and in 2 cases it was paternal in origin. Seven cases, including both those with an additional paternal chromosome, appeared to be the result of a postzygotic error. In contrast to the situation in nondisjunction involving chromosomes 21 and X, there was no evidence for nullochiasmate nondisjunction. 44 refs., 1 fig., 2 tabs.