Ring XY bivalent: a new phenomenon at metaphase I of meiosis in man.

The unusual appearance of a ring XY bivalent at metaphase I of meiosis is reported in some cells of an oligospermic human male. Higher than usual frequencies of ring configuration in the XY pair were also observed during prophase I. The defect could be attributable to loss of some DNA sequences from the distal heterochromatic tip of the Y chromosome long arm.