Infertile mares with XO and XY sex chromosome deviations

In horses, small ovaries and failure to cycle during the breeding season suggest abnormalities in the sex chromosomes (Chandley et al. 1975; Long 1988). Horses with sex chromosome deviations often reach maturity, because foals with sex chromosome defects have less severe abnormalities than those with autosomal deviations (Bowling and Millon 1990). Therefore, the connection between infertility and sex chromosome alterations in horses is possible to detect. Mares with XO and XY sex chromosomes usually do not show normal oestrous behaviour. However, mares showing irregular oestrous signs have been reported, and many affected mares have been able to be bred because a lack of progesterone production leads to a passive acceptance of the stallion (Miyake et al. 1979; Long 1988; Bowling and Hughes 1993; Mäkinen et al. 1999, 2001). Their external reproductive organs are normal, and on transrectal examination these mares are impossible to distinguish (Chandley et al. 1975). In most cases, owners of such mares are not aware of the condition until puberty, when the mares with XO or XY sex chromosomes fail to cycle. In this report, we describe the clinical and genetic examinations and necropsy findings in 2 infertile mares, referred for further studies by veterinary surgeons experienced in equine practice.