Sex Disorders in Horses and Humans

Could Your Mare be a Stallion? Did you know that mammals of all types (not just horses) can appear female on the outside, but are actually genetically male? Within the gender/sex disorder category, there is a wide range of genetic variety that can result in confusion and surprise. Read on to see an example of what we've experienced in our lab:

Scenario - A client sends in sample of pulled horse hair for DNA testing. They fill out the information online and declare that their horse is a bay mare, standing at 16h with a bit of an attitude and a somewhat "cresty" neck. When the DNA is run on the sequencing platform, the genetic testing is reviewed and flagged for "an inconsistency." The aforementioned "mare" is testing in a specific region for "XY" instead of "XX"...odd. Since the female sex is genetically "XX" and males are "XY" something isn't adding up.

A call is made by our client services team to double-check and alert the owner. Was there possibly a mistake in reporting the observed gender of their horse? "Nope" is the reply, and the next interesting comment is made, "We are testing because she doesn't seem to want to cycle or breed and we can't understand why". Taking a breath, we have to let them know that, while their big, beautiful aggressive mare is probably healthy (no known genetic diseases identified), that they may have a hard time breeding her because...well...she's a male. Our next question to the client is to ask about a veterinary exam, or, "Have you had a vet check 'the plumbing?'"

Upon physical exam and ultrasound, many times the findings can include anything from malformed or no ovaries, to surprise testes, to hormone producing tumors, to no sex organs at all!

What's going on here??!

There are a wide variety of genetic sex disorders that can appear in horses and many mammals, including humans. It's estimated that about 1 in every 1600-2000 horses may be affected by the disorders we are about to explore. That means it's rare, but it does happen (and more often than we were prepared to see!) Below are four such disorders with descriptions and links to help illustrate the effects and reach of these genes/chromosomes. Read through these and try to decide which one of these may apply to the above scenario:

Klinefelter Syndrome (XXY) - Klinefelter's mammals are genetically male with one extra "X" chromosome. These individuals experience delayed puberty, reduction in size of testes and are often missing or light on secondary sex characteristics (such as facial hair in humans or neck crest in stallions).

Testicular Feminization - In this case an XY male does not respond to androgens and, therefore does not develop male sex characteristics. Sometimes this is due to a missing or partial "Y." Nature defaults to the female phenotype and thus, the animal will appear female; albeit with a wide variety of internal reproductive anatomy and likely sterility.

Turner Syndrome (XO) - Affecting only females, one of the X chromosomes or part of an X chromosome is missing. These mammals are described as having shorter stature, delayed puberty and infertility. Depending on the mammal, there may be physical signs (such as in humans where neck webbing and lower ear set is observed).

Jacob's Syndrome or "Superman" Syndrome (XYY) - Another aneuploid genetic condition, these animals will have an extra Y or part of an extra Y. In humans, characteristics include increased height, length of arms, and sometimes an increase in anxiety or respiratory problems have been reported. It was previously believed that human affected with Jacob's Syndrome were disproportionally aggressive and, thus, were more likely to be imprisoned. This believe has since been disproven. It's worth noting, however, that breeding stock animals with XYY will, statistically, produce more male than female offspring as they have only a 1 in 3 chance of passing on the "X" to their offspring!

There are additional sex and gender disorders (and probably several yet to be discovered) beyond chromosomal count which can lead to issues and surprises in our horses. An example of this might be Androgen Insensitivity Syndrome. In this instance, the genetic mutation affects the animals' ability to respond to male hormones during development. The result can be a foal that appears female, partially male, delayed in development or all of the above. Some of these things can be assisted with surgery or hormone therapy, however, there is no proof to date that it should affect your ride. Horses with these disorders are typically otherwise normal and, aside from the occasional odd behavior and the discoveries upon ultrasound, one may never know the difference.

Do you or your veterinarian suspect your horse may have a case of chromosomal sex disorder? Reach out to us and share your story! Don't forget, genetic testing is a simple, quick and painless way to check your horse for issues, health problems and even talents or abilities. Know your horse, all the way down to the DNA.