RESUMEN
A 9-year-old Thoroughbred mare with normal external genitalia and regular oestrus symptoms was gynecologically examined prior to insemination. This primary examination revealed the presence of a hypoplastic uterus and the lack of normal ovaries, and the mare was therefore subjected to more detailed diagnostics, including endocrinological, genetic, and clinical tests. Diagnostic imaging with the use of ultrasonography and endoscopy confirmed the underdevelopment of internal genitalia. Analysis of circulating sex hormones revealed very low concentrations of progesterone and oestradiol. Finally, cytogenetic analysis showed the presence of non-mosaic X trisomy (65,XXX), an aneuploidy of sex chromosomes that is rarely detected in horses. This finding was also confirmed by molecular methods, including highly sensitive droplet digital PCR (ddPCR) and microsatellite markers genotyping. Our study reveals the need for gynaecological and genetic evaluation of broodmares, even if their phenotype (including developed external genitalia and oestrus symptoms) shows no signs of potential abnormalities.