DNA tests for inherited diseases in horses
A DNA test can reveal that a horse carries a variant responsible for an inherited disease, even when there are no visible signs. This information can help explain a disease, guide the horse’s care or inform breeding decisions. To use it well, you need to choose the right test and understand exactly what its result can tell you.
What a DNA test looks for in a horse’s genes
A gene is a stretch of DNA involved in how the body functions. A variant is a difference in the sequence of that DNA. Some variants cause disease, others only increase the risk of developing it, and many have no known effect on health. A test for an inherited disease looks for one or more variants linked to that disease. The strength of the evidence for that link needs to be considered.
In the examples on this page, the genes being tested are autosomal: they are on chromosomes other than the sex chromosomes. A horse usually has two copies of each, one inherited from its dam and one from its sire. The laboratory determines whether the variant being tested for is absent from both copies, present in one or present in both. This combination is part of the horse’s genotype.
Testing may be requested before breeding to identify a variant that could be passed on, or in a horse with signs of illness to investigate a specific cause. These tests also differ from parentage tests, which compare DNA markers to verify the reported parents. A parentage test, a coat color test and a disease test answer different questions, even though they all use DNA.
Choosing DNA tests for a horse’s ancestry and the question you need to answer
Start with a specific question: are you trying to explain muscle problems, check for a variant already identified in the family or understand the risks of a planned mating? A veterinarian can consider this question alongside the signs observed, other test findings and the horse’s family history. Ordering many DNA tests for a horse with signs of illness without this context can leave other possible causes unexplored.
Breed and bloodline can help guide which variants to test for, because some have been reported more often in particular populations. Neither can establish an individual horse’s result on its own. A crossbred horse may also have inherited a variant from its ancestors. When a variant has been identified in the family, its name and the parent’s laboratory report provide more precise information than breed alone.
A panel combines several tests in a single order. Before choosing one, check the exact list of diseases and variants included: panels with similar names can have different contents. A negative panel result applies to that list; it does not cover every genetic disease in horses. If you need the result for an official purpose, first check the relevant organization’s requirements for the laboratory, sample collection and verification of the horse’s identity.
Checking the scientific evidence behind a horse’s DNA test
Detecting a variant accurately and establishing its link to disease are two different questions. Analytical reliability concerns the laboratory’s ability to read the DNA it is testing. Clinical validity concerns what that finding actually tells us about the disease: is the variant an established cause, a confirmed risk factor or a possibility still under investigation? A technically accurate result may offer little useful health information if the link has not been demonstrated.
To assess a test, look for a description of the variant, published research linking it to the disease and the populations in which it has been studied. Also check whether the laboratory tests directly for the disease-associated variant or for a nearby marker, a recognizable point in the DNA associated with it. During inheritance, a nearby marker can sometimes be separated from the disease-causing variant. What an indirect marker can tell you therefore depends on whether that association has been validated in the relevant population.
PSSM1 provides an example of an established test. It looks for a variant in GYS1, a gene involved in storing glycogen, a stored form of sugar in muscle. Commercial tests offered for other muscle diseases, including those marketed as PSSM2 tests, need to be assessed separately. In the Quarter Horses examined in the study cited below, the P2, P3a/P3b and P4 variants did not reliably distinguish horses with PSSM2 from control horses. That conclusion applies to those variants and the population studied. Validation of a PSSM1 test therefore does not, by extension, validate another test with a similar name.
Preparing a DNA sample and making sure it belongs to the horse being tested
Many equine DNA tests use mane or tail hairs pulled out with their roots, because cells at the root end provide the DNA for analysis. Cut hairs are not the sample requested for these tests. Some laboratories also accept blood or other tissues. Before collecting anything, confirm the sample type, amount needed and shipping requirements with your chosen laboratory.
Prepare a separate sample for each horse, label it immediately and avoid mixing hairs from different animals. Follow the laboratory’s instructions for packaging and accompanying paperwork. If several horses are being sampled, matching each animal to the correct form and sample deserves as much care as collecting the sample itself.
A name on an envelope does not, on its own, certify the horse’s identity. For a result intended for a sale, registry or another purpose that requires verified identity, find out which procedure is accepted. Depending on the organization, this may involve checking the microchip, having an authorized person collect the sample or using a particular form. An insufficient or unusable sample calls for a repeat analysis or a new sample; it is not a negative result.
Reading a horse’s DNA result in light of the variant and its inheritance pattern
First check the identity shown on the report, the test name and the variant tested for, then read the laboratory’s key. Notations such as N/N or N/HRD describe the two versions found at the DNA location being studied. In this HERDA report example, N means that the copy does not carry the variant being tested for; it does not mean that the gene is missing. Abbreviations can differ between laboratories.
A horse is heterozygous when its two versions at that location differ, and homozygous when they are the same. These words describe a genetic combination. They do not, on their own, tell you whether the horse has a disease. To understand that, you need to know the variant’s effects and how the disease is inherited.
HERDA is a recessive disease that makes the skin fragile: for the tested variant, a horse with one copy is an unaffected carrier of HERDA. PSSM1 has a dominant inheritance pattern: one copy of the variant can already cause muscle signs. The term unaffected carrier therefore does not automatically apply to every horse with one copy of a variant. Susceptibility also needs to be interpreted within its own limits: a variant may increase risk without making disease certain.
The same number of variant copies has different consequences for HERDA and PSSM1
This comparison covers two separate tests, one for HERDA and one for PSSM1. For each disease, only copies of the variant detected by that disease’s own test are counted. HERDA and PSSM1 variants are not added together.
Copies of the tested variant : No copies
Copies of the tested variant : One copy
Copies of the tested variant : Two copies
These interpretations apply to the variants covered by the two tests shown here. They are not a universal rule for recessive diseases, dominant diseases or genetic susceptibility.
Using the mare’s and stallion’s DNA results before breeding
For a simple recessive disease such as HERDA, you need both parents’ results to assess the possible combinations in their foal. If the mare and stallion each have one copy of the variant responsible for HERDA, each pregnancy has a 25% chance of a foal inheriting two copies and being affected, a 50% chance of a foal inheriting one copy and being an unaffected carrier of HERDA, and a 25% chance of a foal inheriting no copies. These probabilities apply afresh to every pregnancy: four foals will not necessarily follow that distribution.
If one parent has one copy of the HERDA variant and the other has none, the foal cannot inherit two copies of that variant. No foal affected by HERDA is therefore expected from this mating for this variant, but each foal has a 50% chance of being a carrier. This reasoning does not apply to every disease or every variant.
For a dominant disease such as PSSM1, choosing a partner with a negative result does not eliminate the chance of transmission. A parent with one copy of the variant has a 50% chance of passing it to each foal. The chance of inheriting the variant must not be presented as an identical chance of showing signs, because the expression of the disease varies.
Breeding decisions also need to consider the horses’ health and the population’s genetic diversity. The presence of a recessive variant alone does not mean that an entire bloodline must always be excluded. For certain simple recessive diseases, keeping an unaffected carrier in a suitable breeding program may be considered, provided it is mated to partners tested without the variant. This can prevent affected foals while limiting the loss of diversity. The approach requires follow-up of the offspring and must be adapted to the disease and population concerned.
What a negative or positive DNA result cannot tell you about a horse’s health
A negative result means that the variant or variants tested for were not detected in the sample. It does not rule out another variant in the same gene, another genetic disease or a cause unrelated to genetics. When several variants can cause a disease, check which ones were actually included in the analysis.
A positive result does not always describe the horse’s current clinical condition or how its health will develop. Some variants have well-established effects, while others increase susceptibility to signs whose occurrence and severity vary. A veterinarian interprets the result alongside an examination and other available information. Finding a variant is not enough to attribute all of a horse’s signs to it.
Many conditions involve several genes as well as factors such as age, diet or environment. The results of a few targeted tests cannot capture all of these influences. If problems persist despite a negative panel, further clinical investigation is needed; the result does not mean that every inherited or medical cause has been excluded.
Common questions about DNA health tests for horses
Should a horse that appears healthy have a DNA test for inherited diseases?
Testing can be useful, particularly before breeding or if a variant has been identified in the horse’s family. A horse carrying a recessive variant may have no signs of the disease and still pass the variant on. The tests to choose depend on the horse’s ancestry and the purpose of testing, rather than its appearance alone.
Does a negative DNA panel mean a horse is free of all inherited diseases?
No. It means that the variants included in that panel were not detected in the sample. Check the detailed list and keep it with the report: other variants and other diseases may not have been tested for.
Can a foal’s DNA result be inferred from just one parent’s DNA test?
In general, one parent’s result is not enough to establish the foal’s genotype. For a simple autosomal recessive variant, a parent confirmed to have no copies cannot pass it on, so the foal cannot inherit two copies. That does not tell you whether the foal inherited one copy from the other parent. An inferred result requires suitable parental results, verified parentage and acceptance of this approach by the relevant organization.
Does the same inherited disease test need to be repeated every year?
The genotype being tested does not change with age. Repeating a valid test for the same variant every year therefore does not normally provide new information. A check may nevertheless be warranted if the horse’s identity or the sample is uncertain, results disagree or a new test covers other variants. Official procedures may also have their own documentation requirements.
What should I do if two laboratories report different DNA results for the same horse?
First compare the variant name, the key to the notation and the horse’s identity on both reports: different notation does not necessarily mean conflicting results. If the disagreement concerns the same variant in the same horse, contact the laboratories and your veterinarian to arrange verification. This may involve collecting a new sample with the horse’s identity checked.
Key points for using DNA health tests in horses
- Choose a test to answer a specific question, taking the horse’s ancestry and history into account.
- Interpret the number of copies alongside the variant’s effects: one copy has different consequences for HERDA and PSSM1.
- Before breeding, consider both the mare’s and the stallion’s results for the same variant.
- A DNA result tells you about what was tested; examining the horse remains essential to understanding its health.
Sources on DNA tests and inherited diseases in horses
References checked on 5 November 2026. University resources explain the tests and their limits; the PSSM2 study concerns the variants it evaluated. The references for each section identify the information used.
- Finno and Bannasch — Applied equine genetics: uses and limits of DNA tests
- UC Davis — Sample types and collection instructions for DNA testing
- UC Davis — HERDA test: results and recessive inheritance
- UC Davis — PSSM1 test: the GYS1 variant and interpreting results
- Valberg and colleagues — Evaluation of commercial variants proposed for PSSM2 diagnosis in Quarter Horses
- IFCE — DNA typing, identification and parentage verification
- IFCE — Genetic diversity and breeding decisions
- University of Minnesota — Simple and complex genetic diseases in horses