Sex reversalSex reversal is a condition where there is a disagreement between the karyotypic sex and the phenotypic sex. Chowdhary reports
that more than 120 such cases have been reported in the horse. XY individuals are expected to be males, but in sex reversal
they appear more like females. This is the second most common abnormality in infertile mares.
XX sex reversal is far less common but does occur. Individuals with these conditions appear mostly as female with varying
degrees of external male-like sex characteristics. The external genitalia may be normal, but there may be intra-abdominal
testis, ovaries or both.
These abnormalities are of interest in that they may explain some clinical or behavioral problems that have occurred in horses.
It is apparent that more karyotyping should be done and that this may then give us a clearer picture of these special individuals
and their true percentages in the general population.
In reality though, once the genetics of a horse have been determined, there is little that can be done clinically as far as
"One can try hormonal therapy with these horses," says Chowdhary, "but they are unlikely to show much response."
Clients' perceptionsSome clients are also unwilling to challenge their perception of their horse's gender.
This tends to occur more often with horses kept as companions but it is surprising in some professionals and broodmare owners
Rena's owners were convinced that "she" was a mare and had been relating to her as such. Even with all the information that
was uncovered during the physical exam, ultrasound and hormonal analysis, they decided that she was a she. Since breeding
was not an issue, they did not wish to pursue the diagnosis any further.
Dr. Marcella, a 1983 graduate of Cornell University's veterinary college, was a professor of comparative medicine at the University
of Virginia. His interests include muscle problems in sport horses, rehabilitation and other performance issues.