Veterinary researchers have taken a leap forward in explaining XY sex reversal, the most common equine sex chromosome abnormality. A team of scientists from the University of Kentucky's Gluck Equine Research Center and the College of Veterinary Science at Texas A&M University have been examining the Y chromosome to determine what prompts a genetic mutation that causes seemingly healthy mares to be infertile.

XY sex reversal is the development of female genitalia in a horse possessing the male Y chromosome; i.e., the horse should have displayed male genitalia. When an abnormality affects sex differentiation, the developing equine fetus "defaults" to the female developmental pathway.

"The majority of affected horses have normal female genitalia and no stallionlike behaviors," said Teri L. Lear, PhD, an immunogenetics researcher in the molecular cytogenetics laboratory at the Gluck Center. "The sex reversal is often not noted unless there is a problem with infertility."

Some "XY mares," as they are called, however, show various degrees of masculinization, visible as abnormally developed reproductive tracts