Y chromosome heterochromatin of differing lengths in two cell populations of the same individual

Objective To present a prenatal diagnosis report on a case where G‐banding analysis of fetal metaphase chromosomes showed populations of cells with two different Y chromosomes; one with a short block of heterochromatin (Yqh−) and one with a longer block of heterochromatin (Yqh+). Methods These two populations of the Y chromosome were studied using fluorescent quinacrine banding and fluorescent in situ hybridization (FISH). A chromosome paint corresponding to the euchromatic region of the Y chromosome, and probes corresponding to the SRY, DYZ1, and DYZ3 regions were used for this study. Results Both Y chromosomes appeared to be structurally normal by these analyses. Subsequent ultrasound examination at 20 weeks' gestation revealed normal male genitalia. Follow‐up with a neonatal blood sample analysis confirmed the above findings. Conclusions This study reports a direct prenatal diagnosis case of two populations of the Y chromosome in the same individual. This apparent mosaicism may be explained by a postzygotic simple deletion or unequal crossover event between sister chromatids in the DYZ region. Copyright © 2005 John Wiley & Sons, Ltd.