Five- Panel Testing

Learn details about modern genetic testing and its positive effect on breeding decisions.

the Quarter Horse world, as well as breeds crossed with Quarter Horses. Horses suffering muscle tremors, weakness, collapse, and even death made headlines when the cause was traced to a genetic disease attributed to a prominent Quarter Horse bloodline. When researchers announced that HYPP (hyperkalemic periodic paralysis) linked to one very famous stallion named Impressive, there was an uproar in breeding barns across the country.

A decade later breeders began to accept the importance of reducing the spread of this deadly disease. In 1998, the American Quarter Horse Association adopted a rule requiring all foals descending from Impressive to be tested for HYPP, with results listed on their registration papers. In 2007, horses carrying two genes for this disease were no longer accepted for registration. This marked the beginning of genetic testing and AQHA’s efforts to minimize the spread of genetic diseases within the breed.

Fast-forward to 2016. Genetic tests are now available for five different heritable diseases that’ve been identified in Quarter Horses and other stock breeds.

Here, I’ll explain what the five-panel genetic test is all about. First, I’ll give you a primer on basic genetics so you can understand how genes are inherited. Then, I’ll teach you the basics about the five different diseases that make up the fivepanel genetic test. With this information, you’ll better understand how testing can help you make informed breeding decisions.

Genetics 101

When egg and sperm get together to create a foal, there’s a lot going on behind the scenes. Size, color, temperament—even some aspects of health—are predetermined by the way traits passed from the mare and stallion combine. The science behind all of this is called genetics, and it’s a complex, fascinating process. Here’s how it works.

Every horse has, within each cell, 32 pairs of chromosomes that contain all of the genetic information that makes him what he is. One set of these pairs came from his dam via the egg, the other from the stallion via the sperm. These chromosomes carry more than 30,000 genes, or specific messages, that determine different traits.

Genes can be either dominant (they’re expressed even if carried on only one set of chromosomes) or recessive (a matching pair must be present to have the trait in question). Some genes have incomplete dominance, meaning the trait is exhibited most strongly if both sets of genes are present, but can still be present even if there’s only one gene there. Here’s an example of how it works.

Pretend that the trait for patience is a dominant trait—symbolized by a capital P. A small p means your stallion (or

mare) doesn’t carry the patience gene. Your stallion gets one gene from his dam, and one from his sire. The following combinations can result. Combination 1: PP. Your stallion got a patience gene from each parent, so is homozygous for patience. Not only will he stand in the crossties for hours without complaint, but if he’s bred, he’ll pass on a patience gene to his offspring, too. Combination 2: Pp. Your stallion got a patience gene from one parent, but not from the other. He’s heterozygous for patience. Because the gene is dominant, he’ll still be patient. If the patience gene had incomplete dominance, he’d be patient—just not quite as patient as he would be if he were PP. If he’s bred, only half of his offspring will get the patience gene from him. The other half will depend on their dams to determine whether they’ll be patient. Combination 3: pp. Your stallion is homozygous recessive when it comes to the patience gene. He’s not patient at all, and his offspring will only be patient if they inherit a patience gene from their dams.

Now let’s change our scenario, and pretend that the patience gene is recessive—meaning it’s only expressed if both genes are present. In this situation “p” would stand for patience, and your stallion will only be patient with Combination 3 (pp). Although he’d carry the gene in Combination 2 (Pp), no one would know he had it, because he wouldn’t be patient at all. Half of his offspring would inherit the gene, but they’d only be patient if they inherited another patience gene from their dams (making them pp). And if your stallion were PP, there’s no chance any of his offspring would be patient. Now let’s take a look at the five heritable diseases identified in stock breeds and tested for with the five-panel genetic test. These include hyperkalemic periodic paralysis (HYPP), polysaccharide storage myopathy (PSSM), glycogen branching enzyme disease (GBED), hereditary equine regional dermal asthenia (HERDA), and malignant hyperthermia (MH). This disease causes a dysfunction of the chan- nels, or pathways, that sodium passes through into and out of muscle cells. This disrupts the conduction of impulses that stimulate muscle contraction and can lead to episodes of muscle tremors, weakness, cramping, and collapse. In severe cases, an HYPP episode can be fatal.

How is it inherited? HYPP is a dominant gene, with incomplete dominance. This means 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). If he’s homozygous for the trait, all of his offspring will have the disease—but its severity will depend on whether they inherit a gene from their dam. If he’s heterozygous, he’ll only pass on the gene to 50 percent of his offspring.

How common is it? While estimates say that only 1.5 percent of Quarter Horses carry this gene, its incidence in halter horse bloodlines is a staggering 56 percent.