Fertile male mice with three sex chromosomes: evidence that infertility in XYY male mice is an effect of two Y chromosomes.

In the mouse XYY males are sterile, presumably because pairing abnormalities resulting from the presence of three sex chromosomes lead to meiotic breakdown. We have produced male mice, designated XYYX, that have three sex chromosomes pairing regions but only one intact Y chromosome. Unexpectedly XYYX, males are fertile, although they are no more efficient in sex chromosome pairing than previously reported XYY males. We conclude that the sterility of XYY males is caused by a combination of the deleterious effect of two Y chromosomes, presumably acting prior to meiosis, and pairing abnormalities resulting in significant meiotic disruption.