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 XYY*X, that have three sex chromosomes pairing regions but only one intact Y chromosome. Unexpectedly XYY*X, 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.