Meiotic consequences of an intrachromosomal insertion of chromosome No 1: a family pedigree.

In three generations of the proband's patrilineal relatives, 14 subjects were found to be carriers of a "shift" insertional chromosome No. 1 (46XX or XY, ins(1)(p32q25q31)). The proband and three female relatives, who were mild to moderate mental retardates with minor congenital anomalies, were trisomic for the insertional segment, (1)q25q31. Another subject, who was a markedly immature female abortus with congenital abnormalities, was found to be monosomic for this same chromosomal segment. The cytogenetic evidence suggests that each of these unbalanced recombinant progeny was the result of a single crossing over in the noninsertional loop of a paternal pachytene bivalent of the balanced insertional chromosome No. 1.