AQHA Six-Panel Genetic Testing

If you are breeding Quarter Horses, you have probably come across the term "six-panel," or seen a stallion advertised as "six-panel N/N." In this article we are going to get into why this is important for this breed in particular.

What the six-panel covers

Here is where this gets a little more complicated: these six conditions are not all inherited in the same way. How each condition works in terms of heritability matters, as a carrier of a recessive condition may be safely bred to a non-carrier, while breeding two carriers together can create a risk of producing an affected foal. Understanding the genetic status of both the mare and the stallion allows breeders to make much more informed mating decisions.

For experienced Quarter Horse breeders, genetic testing has become a routine part of breeding decisions. They are not afraid of it, as they know it actually gives them the ability to make informed and responsible breeding decisions. For someone new to the breed, or to breeding, however, the terminology can be confusing.

What exactly is the six-panel test? What diseases does it test for? Does a horse that carries one of these mutations need to be excluded from a breeding program? And why is it important to know the results before choosing a mating?

Why do Quarter Horse breeders test?

Genetic testing gives breeders information they cannot get simply by looking at a horse. A horse can appear completely healthy and still carry a genetic variant associated with an inherited disease. This is particularly important with recessive conditions, where a carrier may show no signs of disease but can produce an affected foal if bred to another carrier.

Distinguishing between dominant and recessive inherited disorders is key to making a safe pairing, and genetic testing is how breeders do that. For breeders, the objective isn't necessarily to eliminate every horse carrying a genetic variant. Rather, it is to understand the genetic status of a mare and a stallion, and therefore know what the possible outcomes of the match will be.

The six conditions explained

1. HYPP — Hyperkalemic Periodic Paralysis

HYPP is associated with a mutation affecting the sodium channel in muscle cells and is particularly associated with Quarter Horses descending from the Impressive bloodline. Affected horses can experience episodes involving muscle tremors, weakness and paralysis. Severe episodes can potentially be fatal. HYPP is inherited as a dominant trait, meaning that a horse with one copy of the mutation can inherit and pass on the condition. Clinical signs can vary between horses.

AQHA records can identify HYPP results as N/N, N/H or H/H, and AQHA has specific requirements relating to HYPP testing in certain horses and bloodlines. For a breeder, knowing the HYPP status of both potential parents is therefore particularly important.

2. PSSM1 — Polysaccharide Storage Myopathy Type 1

PSSM1 affects the way muscle stores and uses glycogen. The condition is particularly relevant to Quarter Horses but also affects other horse breeds, so is commonly tested for in many breeds. PSSM1 can be associated with muscle stiffness, exercise intolerance and episodes of rhabdomyolysis, commonly described as "tying up."

PSSM1 is associated with a mutation in the GYS1 gene. According to the AAEP, the mutation is inherited as a dominant trait, although the severity of clinical signs can vary considerably, and homozygous horses can be more severely affected than heterozygotes.

With PSSM1, a positive genetic result does not tell you exactly how severely a particular horse will be affected. It does, however, give you valuable information about the horse's genetic status and potential offspring.

3. MH — Malignant Hyperthermia

Malignant hyperthermia is associated with a mutation in the RYR1 gene and can result in a potentially life-threatening reaction involving abnormal muscle metabolism and body temperature. The AAEP describes the condition as inherited as a dominant trait. Genetic testing can identify horses carrying the mutation.

Knowing a horse's status can therefore be important not only when making breeding decisions but also when veterinarians are planning procedures involving anesthesia.

4. GBED — Glycogen Branching Enzyme Deficiency

GBED is quite different from the dominant conditions discussed above. It is an autosomal recessive disorder associated with a mutation affecting glycogen branching enzyme. A horse that carries one copy of the mutation can appear completely normal. The problem arises when two carriers are bred together.

A foal that inherits two copies can be severely affected and generally does not survive. This is one of the clearest examples of why genetic testing can be useful even when both parents appear perfectly healthy.

If a mare is a carrier, for example, knowing the stallion's GBED status allows the breeder to avoid producing an affected foal by choosing a stallion that does not carry the mutation.

5. HERDA — Hereditary Equine Regional Dermal Asthenia

HERDA is an inherited connective-tissue disorder. It affects the horse's skin and other connective tissues. Affected horses can develop extremely fragile, loose skin and serious wounds and scarring, often becoming apparent when horses begin training or are subjected to relatively minor trauma.

HERDA is autosomal recessive, which means carriers may look completely normal. The significance for breeders is therefore straightforward: two carriers can produce an affected foal.

A review published through Equine Veterinary Education noted that the carrier frequency can be substantially higher in some Quarter Horse populations, and specifically recommended testing Quarter Horse mares and horses from associated bloodlines as well as stallions.

6. MYHM — Myosin-Heavy Chain Myopathy

MYHM is the most recent addition to the AQHA genetic health panel. It is associated with a variant in the MYH1 gene and can be associated with two different clinical presentations:

• immune-mediated myositis, which can cause substantial muscle atrophy; and
• non-exertional rhabdomyolysis, which can cause muscle damage and associated clinical signs.

The inheritance pattern is described by the AAEP as autosomal co-dominant with variable penetrance. In other words, carrying the variant does not necessarily mean that every horse will develop the same clinical signs or severity.

Dominant versus recessive

With a recessive condition, two carriers may be perfectly healthy themselves but can produce an affected foal. With a dominant condition, one copy of the mutation can be enough for the horse to be considered genetically affected.

For a recessive condition such as HERDA or GBED, a carrier is not necessarily unusable as a breeding animal. Instead, the breeder can avoid breeding that carrier to another carrier. This is where genetic testing — and breeders being transparent about the results — is key, as it allows otherwise genetically healthy, good breeding stock to be responsibly bred.

For example:

Carrier mare × N/N stallion

can be a very different proposition from:

Carrier mare × carrier stallion.

The first mating cannot produce a homozygous affected foal for that recessive mutation. The second can.

Does a carrier need to be removed from a breeding program?

For recessive diseases in particular, responsible breeding can involve using a carrier horse with a non-carrier mate and then making informed decisions about the resulting offspring.

The alternative — never testing, and unknowingly breeding carriers together — can create an entirely avoidable risk. For recessive conditions in particular, genetic testing gives breeders more information and more options when making breeding decisions, rather than automatically meaning that a carrier must be removed from a breeding program.

What does this mean for mare owners?

Imagine that you own a Quarter Horse mare and have narrowed your search down to three stallions. All three have good conformation, performance records, pedigrees and breeding records, but their genetic results are different.

One may be N/N across the six-panel. Another may carry one of the recessive variants. A third may have a result that requires particular consideration because of the inheritance pattern of the condition.

Without genetic testing, those differences are invisible. With genetic testing results, you can make an informed decision for your mare. This is particularly useful when a mare comes from a bloodline in which a particular inherited condition is known to occur.

AQHA's current registration guide states that the sire must have a Genetic Health Panel and DNA type on file. AQHA also has specific genetic-testing requirements that apply in particular circumstances, including certain bloodlines and breeding situations.

The practical result is that genetic information is now readily available as part of the AQHA breeding and registration system, making it much easier for breeders to incorporate genetic status into mating decisions.

Genetic tests in StallionApp

StallionApp allows mare owners and stallion owners to record genetic testing results on their horse's profile. This means that other users who might be considering breeding a horse can clearly see the genetic profile and make a more informed decision.

Once genetic results have been recorded, StallionApp's test mating functionality can use that information to identify potential genetic risks associated with a pairing. This allows breeders to consider genetic compatibility alongside pedigree, conformation, performance and other factors when evaluating a potential mating.

Keeping those results on the horse's record rather than in a folder somewhere is what makes them usable at the moment you are actually choosing a stallion — which is the same argument for keeping thorough mare breeding records generally.