Evidence for the repeated primary non-disjunction of chromosome 21 as a result of premature centromere division (PCD).

A clinically normal 28-year-old woman had three conceptuses with trisomy 21 and one normal child. She showed minimal cytogenetic evidence of mosaicism: 4% of her blood cells and 6% of skin fibroblasts had trisomy 21. Also, 7% of her blood cells showed aneuploidy of the X chromosome which was associated with premature centromere division (PCD,X); 6% of fibroblasts showed trisomy 18, 10% of fibroblasts showed PCD,21, and 1% PCD,18. It is unlikely that this woman is a constitutional mosaic for trisomies X, 18, and 21, all at low levels. We suggest that she has a predisposition to irregular centromere separation and that chromosomes X, 18, and 21 are most susceptible to its action.