Researchers know that a maternally inherited sex development disorder (DSD) can cause repeated cases of infertility in a single family of horses. But they’ve recently identified the gene responsible for the condition and have begun testing for it to help breeders make better mating choices.

Androgen receptor (AR) gene mutations can make a male fetus develop as a structurally sound filly—at least on the outside. Internally, these XY-chromosome horses (the chromosome for males) lack a uterus and ovaries and sometimes have malformed testes, said Tamas Révay, PhD, of the Department of Biomedical Sciences at the University of Guelph, in Ontario, Canada.

Last year, Spanish and Polish researchers developed a test for this “XY mare chromosome abnormality” based on a deletion in the SRY gene, which is found on the Y chromosome (coming from the sire). However, this SRY-gene abnormality—which appears to be spontaneous as opposed to hereditary—is just one possible cause for this DSD.

“The AR gene mutation is causing a hormonal problem, as opposed to a transcription problem in the SRY gene mutation,” said W. Allan King, PhD, also of the University of Guelph Department of Biomedical Sciences. The SRY gene manages the structural development of the ovaries and testes, whereas the AR gene manages the use of those gonads as they release hormones and cause the rest of the reproductive structures to develop. “It’s a different process,” he told The Horse