Objective: To examine sperm meiotic segregation in a man with mosaic ring chromosome 21.
Design: Case report.
Setting: Hospital departments of reproductive biology, cytogenetics, gynecology, and obstetrics.
Patient(s): One patient referred for cryptozoospermia, heterozygous for a ring chromosome.
Intervention(s): Fluorescence in situ hybridization with chromosome 21-specific probes after sperm selection.
Result(s): A total of 169 spermatozoa were selected; 92.3% carried a normal 21 chromosome, 6.5% the ring chromosome, and 1.2% both.
Conclusion(s): Ring chromosome frequency in mature sperm cells was low and may be due to preferential meiosis of normal spermatogonia,which could explain the cryptozoospermia and unexpected ratio in this case.