The owner of Koko, a Quarter horse at a farm just outside Kingston, began to observe she was acting very strange for a mare…
The horse would become excessively aggressive and also mount other mares just like a stallion. After a series of tests, the Ontario Veterinary College – who the owner contacted after the strange behaviour continued – determined it was the first identified family of horses with pseudo-hermaphroditism, a rare genetic anomaly that causes genetically male horses to appear female.
University of Guelph professors W. Allan King, in the Department of Biomedical Sciences, and Tracey Chenier, in the Department of Population Medicine, report mares that act like stallions are not that unusual. It can often indicate the presence of an ovarian tumour, which could cause heightened levels of the male hormone testosterone.
But when Koko’s reproductive system was examined, it was discovered Koko had internal testes. “He” was a “she”.
Since there were both internal male and external female genitalia, a karyotype analysis was carried out by the University of Guelph researchers (a detailed study of its chromosomes) to ultimately decide Koko’s gender. The results revealed the horse did have XY male chromosomes, and later they determined two other suspected relatives – traced back to the same female ancestor as Koko – also display the male genotype characteristics.
Professor Chenier was quoted as saying “It was a very exciting discovery to be expecting a tumour but discover an entirely different internal system than anticipated… and then to identify the same problem in three close relatives was very unexpected.”
Since these three horses have the same ancestor, researchers said the condition is certainly a genetically inheritable one.
Researchers report that a similar condition can occur in humans and is linked recessively to the X chromosome. The condition, known as Androgen Insensitivity Syndrome, is also passed down through the female line.
Photo based on source by Iirraa