Horses with XY sex chromosomes are always male, right? Well, maybe not. Spanish and Polish researchers have determined that sterile mares could actually have male chromosomes. And new diagnostic procedures could be making this syndrome easier to detect, resulting in less hassle and expense for breeders.

The “XY mare” chromosomal abnormality probably goes frequently undiagnosed, mainly because these mares usually have anatomical structures similar to XX mares, said Sebastián E. Demyda-Peyrás, MSc, PhD, of the Department of Genetics at the University of Córdoba, in Spain. But it could be a prevalent condition, as horses are known to have more chromosomal abnormalities (95% of the time in the sex chromosomes) than other domestic species.

“These mares usually look normal, even if they sometimes show some masculine behaviors like trying to mount another mare,” Demyda-Peyrás said. “But generally speaking, you wouldn’t know that your mare had male sex chromosomes.”

Chromosomal testing by karyotyping (which examines the number and appearance of chromosomes in a cell sample) is more complicated in horses than in humans, he added. So checking for XY chromosomes hasn’t traditionally been a first-line diagnostic procedure in sterile mares. Working on a molecular level with DNA from hair and blood, however, could lead to easy and inexpensive results