Molecular studies of the aetiology of trisomy 8 in spontaneous abortions and the liveborn population.

We have determined the parental origin in eight cases of constitutional trisomy+ 8. In all four cases of spontaneous abortion, the additional chromosome was maternal in origin and there was evidence for nullichiasmate meiosis I occurring in the genesis of this trisomy. In contrast, all four cases of liveborn trisomy 8 appear to have arisen by a mechanism consistent with the post-zygotic mitotic gain of the additional chromosome.