Genetic Conditions and Testing

Written By: Pat Griffin, DVM, PhD, DACT

Written By: Pat Griffin, DVM, PhD, DACT

We started the genetics conversation with the basics of genetics, the history of genetic testing, and where we’re going next. We also conducted a seminar for our clients (video at the bottom) to discuss the basics as well as some of the genetic diseases that affect our horses. Now we are diving into an overview of the different types of genetic diseases, and a few examples of each. A lot of the diseases you know of actually have a genetic basis.

Autosomal Dominant Diseases

There are eleven mutations that are currently commercially tested for in the equine industry. Of the autosomal dominant diseases, you have Hyperkalemic Periodic Paralysis (HYPP), Polysaccharide Storage Myopathy (PSM), Malignant Hyperthermia (MH), Grey Horse Melanoma, and Recurrent Exertional Rhabdomylosis (RER).

  • Hyperkalemic Periodic Paralysis (HYPP)

    • Largely affecting the American Quarter Horse and American Paint Horse, HYPP is a disease which makes horses susceptible to unexpected paralysis or muscle tremors. This disease is directly associated with a single line of American Quarter Horses. AQH breeders look for the heavily muscled horses, breed to that line, and can end up with a horse with a severe genetic disease. HYPP also affects heterozygous horses, and can be heavily modulated by environmental causes. This disease is often mistaken for “tying-up” given the similar outward appearance.

  • Polysaccharide Storage Myopathy (PSM)

    • PSM is more widespread, affecting American Quarter horses, Warmbloods (a group that is in general pretty genetically diverse), and draft breeds, among others. This muscle condition also has symptoms similar to tying-up, with muscle stiffness, shifting lameness, and muscle tremors happening in bouts.

  • Malignant Hyperthermia (MH)

    • Mostly affecting American Quarter Horses, this is another muscle issue that is also closely associated with PSM. All three of these muscle conditions affecting Quarter Horses (PSM, HYPP and MH) are included on the five required tests for AQHA registration.

  • Grey Horse Melanoma

    • Not restricted to certain breeds, Grey Horse Melanoma is a color associated disease. We’ve found that there’s an increased rate of greying in horses who are homozygous for this mutated allele, and an increased rate of melanomas to go along with it. It’s very rare to see a teenagaged grey horse that doesn’t have some melanoma. The horses also develop those melanomas at a younger age.

  • Recurrent Exertional Rhabdomylosis (RER)

    • Common in Thoroughbreds, Standardbreds, and Arabians, this is an autosomal dominant disease that is also modulated by the environment. This is another tying-up type syndrome. The environmental stressors can have a large factor in how RER manifests.

Autosomal Recessive Diseases

Of the autosomal recessive diseases, we have Hereditary Equine Regional Dermal Asthenia (HERDA), Lavendar Foal Disease, Cerebellar Abiotrophy (CA), Glycogen Branching Enzyme Deficiency (GBAD), Severe Combined Immunodeficiency (SCID), and Junctional Epidermolysis Bullosa (JEB). 

  • Hereditary Equine Regional Dermal Asthenia (HERDA)

    • HERDA presents in American Quarter horses, generally around two to three years of age, or when you start a horse under saddle. This is indicated by hyper-extensible skin that begins sloughing off. These horses can’t be used as riding horses or breeding horses, and are generally humanely euthanized. This is also on the AQHA list of required genetic tests.

  • Lavender Foal Disease

    • This rare disease is found in the Arabian breed. These foals have a lavender tint to their coat when born, and are very weak. These foals do not survive long past birth.

  • Severe Combined Immunodeficiency (SCID)

    • This autosomal recessive disease is found in Arabians. The carriers, or heterozygote horses, are also affected, but present with a higher incidence of sarcoids. Homozygous foals present with a severely compromised immune system, and usually die from infection before their first year.

  • Cerebellar Abiotrophy (CA)

    • Another condition found almost exclusively in Arabians, most Arabian breeders are on the lookout for CA and SCIDs when breeding now. This is a neurologic condition indicated by a lack of balance in foals. They are unsafe to be ridden, and are often a danger to themselves as they frequently fall and are unable to stand again.

  • Glycogen Branching Enzyme Deficiency (GBED)

    • GBAD can be found in American Quarter Horses and Paint horses. These foals lack the enzyme needed to effectively store sugars, which results in the death of the foal.

  • Junctional Epidermolysis Bullosa (JEB)

    • Saddlebreds, Belgians and other draft type horses can be affected by JEB, a skin disease that becomes too painful for foals to humanely survive. Carriers of the mutated gene present normally, and have no effects of the mutated allele. This is a disease in which the carriers are the problem, as horses affected by JEB do not survive long enough to breed.

Polygenic/Complex Diseases

From a clinical perspective, polygenic, or complex genetic diseases, are more interesting and malleable. There’s an interplay of genes, or genes and environmental factors, that are causing these problems. These are more difficult to characterize and test for than single gene diseases, but also effect more animals. How can we turn these genes off?

  • Recurrent Airway Obstruction (RAO)

    • RAO is a polygenic disease that makes horses genetically prone to heaves and other respiratory issues. These horses are more effected when environmental factors come into play. Horses with RAO are oftentimes fine when they’re out in the pasture, but as soon as they’re stalled, or in a dusty arena, they become symptomatic.

  • Equine Metabolic Syndrome (EMS)

    • EMS is a condition for which we have identified the phenotype, or outward/physical expression. These horses are characterized by obesity, regional adiposity, and laminitis, but we are now working on what the genetic basis for EMS is.

Another type of complex genetic disease is actually the very susceptibility to disease, interestingly. For example, researchers have identified a gene in Thoroughbreds and Arabians that makes them “thin-skinned”, or genetically predisposed to generalized insect bite sensitivity.

It has long been acknowledged that conformation is passed on from parents, but now we’re beginning to look at other musculoskeletal issues that may develop. Thoroughbreds, for example, are particularly prone to tendonitis. There are also certain animals that are prone to developing osteoarthritis (OA) at an early age due to genetics. What genes or genetic interplay is creating these conditions?

Lastly, reproductive success can be linked to genetic ability. Certain lines are far more fertile than others – how can we identify what genes lead to the subfertility affecting other lines?

Role of Color in Genetic Disease

There are many genes that have affects on processes in the body that are totally unrelated. When the color gene mutates, sometimes to give us a color or a pattern that we’re looking for, it can also mutate to develop other issues.

  • Overo Lethal White Syndrome (OLWS)

    • In OLWS foals, the color mutation affects something totally unrelated, the digestive system, and produces a foal with an undeveloped intestinal track. This genetic defect is most prevalent in Paint horses. OLWS is an autosomal recessive disease, so both parents need to have one copy of the bad allele in order to the produce this affect. Because there are no living horses homozygous for this allele, the breeding of the heterozygous carriers of this allele always brings a 25% chance of birthing an OLWS foal, and a 75% risk of producing a new carrier. A simple genetic test of dam and sire could identify the risk in this breeding, and prevent the birth of this foal, which will need to be humanely euthanized as soon as they are born.

  • Multiple Congenital Ocular Anomaly Syndrome (MCOAS)

    • MCOAS is developed from the same gene that produces the silver color for Rocky Mountain horses. Because this is a popular color for the breed, many owners still breed hoping for this color. Though the silver color may be produced, the defect may also be produced. Horses homozygous for this defect will present with a wide range of ocular issues, including iris stromal hypoplasia, abnormal pectinate ligaments, megaloglobus, iridociliary cysts and cataracts. Horses heterozygous for the mutation suffer a less severe variety of ocular issues, largely cysts.


Though there are approximately ten genetic diseases that are commercially tested for today, there are a huge number of genetic diseases that we could test for down the road. As you breed, buy, and care for your horses, consider their genetic background and what’s in the best interest of the horse and the breed.

Questions about the conditions listed above, or other genetic disease? Contact us today or you can reach out to your local veterinarian!

Check out this great video from the genetics seminar hosted for Irongate Equine clients.