A review of cytogenetics in equine reproduction.

The karyotype of the horse consists of 64 chromosomes; 18 pairs have a terminal centromere and 13 pairs a non-terminal centromere. Identification of individual chromosomes is enhanced by the use of band-staining techniques which allow recognition of minor structural rearrangements of chromatin material. Seven previously reported cases of male pseudohermaphroditism with chromosome studies are reviewed. Three were genetic females, one was basically an XX/XY chimaera, one was an XX/XXY chimaera, and one was an XXXY. Also reviewed is an infertile mare with 63 chromosomes and no distinguishable sex chromosomes and a mare with juvenile genitalia and XO/XX sex chromosomes. It is postulated that, in the future, additional chromosome abnormalities will be found associated with maldevelopment of the equine reproductive system and as a cause of early embryonic death and abortion.