Genetic testing lets you make informed decisions to reduce the incidence of these diseases.

stiff, painful muscles and episodes of “tying up” (whole-body muscle cramping).

How is it inherited? Like HYPP, PSSM is a dominant gene, with incomplete dominance. Your horse is likely to be severely affected if he’s homozygous (carries two genes for the trait), and less severely affected if he is heterozygous (carries only one gene for the trait). And if he’s homozygous for PSSM, all of his offspring will have the disease—but its severity will depend on which gene they inherit from their dam. If he’s heterozygous, he’ll only pass on the gene to 50 percent of his offspring.

How common is it? It’s estimated that approximately 10 percent of Quarter Horses carry this gene. In the halter horse bloodlines, its prevalence is 28 percent. dominant with complete dominance, meaning your horse will be equally affected whether he’s homozygous (has two genes) or heterozygous (has only one gene). This pattern of inheritance reduces the chance that unidentified carriers will be responsible for further spread of the disease— although you might never know your horse is affected if he doesn’t experience an episode.

How common is it? The incidence of MH is not yet known.

What Does It All Mean?

The five-panel genetic test helps you not only manage your horse’s individual health, but also make informed breeding decisions that might help reduce the incidence of these diseases. Unfortunately, some of the best performing horses are also carriers of these diseases—and breeding strategies like line-breeding (used to help concentrate desirable traits) also causes these genetic diseases to become more prevalent.

So what decision should you make if you learn that your horse carries one of these genetic diseases? The answer may not be as simple as it seems.

If you’re a purist, with a purely scientific way of thinking, the answer may seem obvious—if your horse carries one of these diseases, don’t breed him. If the carriers are never bred, the disease could be eradicated within a generation.

But what if that carrier also happens to be perfect in every other way? He’s sound, strong, has a super temperament, and is a world-class performer. Aren’t those traits worth preserving? That is the horseman’s dilemma—and why the answer isn’t always black or white.

The middle ground says genetic testing allows breeders to identify what horses are carriers to make smart decisions. Is he an average horse but carries a genetic disease? Don’t breed him. Is he a superstar that’s heterozygous for a recessive disease like HERDA or GBED? Don’t breed him to another carrier, and you can rest assured your foal won’t be affected. (Although it could still be a carrier.) Over time, careful breeding is likely to help gradually reduce the prevalence of the disease, while preserving positive traits.