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A patient with a mitotically unstable dic(Y)(p11) chromosome is reported. Physical examination revealed a small penis with severe hypospadia, undescended testes, rudimetary vagina, uterus, left fallopian tube, and no stigmata of Turner syndrome. Longitudinal chromosome studies over a four-year period, including blood, skin, foreskin, and testicular tissue, revealed 45,X/46,X,dic(Y)(p11)/46,X,del(dic Y) mosaicism. The proportions of these cells varied in the different tissues, and only 45,X and 46,X,del(dic Y) were major cell lines in testicular tissue. Additional minor cell lines were present mainly in peripheral blood: 47,X,dicY,dicY; 47,X,dicY,del(dicY); and 47,X,del(dicY),del(DICY). Premature disjunction of one of the centromeres in a high percentage of the dicentric Y chromosomes in metaphase was observed by Q- and C-banding. Lymphocytes at anaphase and telophase showed lagging Y chromosomes, fragments, and nondisjunction. These observations indicate a high degree of mitotic instability and thus raise the question of the effect of premature centromeric disjunction on mitotic instability of dicentric chromosomes.

A 5-year-old male with mild mental retardation showed a chromosomal rearrangement involving duplication of part of 2q (2q33.3 leads to 2wter) in 70% of metaphases from peripheral blood; the remaining 30% of cells had a rearrangement of chromosome 2 in the form of a ring, viz., r(2)(2p25.2 leads to 2q33.2). Both configurations appeared to be missing a tiny portion of 2p (p25.3 leads to pter). All metaphases examined from cultured skin fibroblasts from the child had the abnormal rod configuration of chromosome 2; none had the ring. The pattern of the 2q duplication/2p deletion in the rod is that expected if there were an inversion in a No. 2 in one of the parents. Q-, G-, and R-banding studies, however, revealed both parents to be chromosomally normal. Furthermore, the finding of an inversion would not explain the origin of the ring. The most probable explanation is that neither parent is mosaic for an inversion, but that the rod and ring configurations arose simultaneously from a de novo, prezygotic or early zygotic exchange in a No. 2, either between complementary DNA strands in G1 or by intrachromosomal exchange in S or G2. Differential selection against cells with the ring chromosome in blood and skin probably occurred during subsequent embryological development. Cytoplasmic malate dehydrogenase (MDH1) was excluded from the terminal band of 2p (i.e., 2p25.3) by deletion mapping.

In women X;Y translocations usually arise as Xp-Yq exchanges. We describe a 17-year-old female with streak gonads, some minor features of Turner's syndrome, and an X;Y translocation involving an exchange between Xq and Yq. Histological examination of the gonads revealed a fibrous stroma with prominent hilar cells. Cultured fibroblasts and peripheral lymphocytes were typed H-Y-. Examination of a buccal smear revealed a single intranuclear structure with the appearance of both a Barr body and a fluorescent Y body. This finding was consistent with the results of BrdU studies showing that the translocation X chromosome had been inactivated in all cells analyzed.