Horse DNA reports use a consistent shorthand of letters and symbols that, once you understand the pattern, are straightforward to read. Most results fall into a few categories: coat color, genetic disease status, parentage verification, and sometimes breed ancestry. Each section of your report follows the same basic logic of listing a gene location, then showing which two versions of that gene your horse carries.
The Basic Format: Loci, Alleles, and Genotypes
Every horse inherits two copies of each gene, one from the sire and one from the dam. On your report, each gene location (called a locus) will show two letters separated by a slash. For example, you might see “E/e” or “A/A” or “N/HRD.” The letter on each side of the slash represents one copy, or allele, of that gene.
When both copies are the same, like “E/E” or “a/a,” the horse is homozygous for that trait. This means the horse can only pass that version to its offspring. When the two copies differ, like “E/e” or “A/a,” the horse is heterozygous, meaning it carries one of each version and could pass either one to a foal. This distinction is the single most important concept for reading any section of your report, because it tells you not just what your horse is, but what it can produce.
Reading Coat Color Results
Coat color results start with two foundational genes: Red Factor and Agouti. These two genes interact to produce the three base coat colors (bay, black, and chestnut), and you need to read them together, not separately.
Red Factor (Extension)
This gene controls whether your horse can produce black pigment at all. The dominant allele is “E” (black pigment possible), and the recessive allele is “e” (only red pigment). A rare third version, “ea,” functions the same as “e.” If your horse’s result is E/E or E/e, it carries at least one copy that allows black pigment. If it’s e/e, the horse can only produce red pigment, making it some shade of chestnut or sorrel regardless of what the Agouti result says.
Agouti
This gene only matters if the horse has at least one “E” allele. Agouti controls where black pigment appears on the body. The dominant allele “A” pushes black pigment to the points (mane, tail, lower legs, ear rims), while the recessive “a” lets black pigment spread uniformly across the entire body. A horse needs two copies of “a” (a/a) to be truly black.
Putting Them Together
Here’s how to combine the two results into a base color:
- Bay: At least one E allele AND at least one A allele (e.g., E/e and A/a)
- Black: At least one E allele AND a/a (e.g., E/E and a/a)
- Chestnut/Sorrel: e/e, regardless of Agouti result
This is where breeding predictions get interesting. Two bay horses that are both E/e and A/a can produce all three base colors: bay (56.25% chance), chestnut (25%), or black (18.75%). But if those same bay parents are both E/E and A/a, a chestnut foal is impossible because there’s no “e” to pass along. The heterozygous or homozygous status of each parent is what determines the range of outcomes.
Dilution Genes and Pattern Modifiers
After base color, your report may list additional genes that modify the base. These include cream, dun, champagne, silver, and pearl dilutions, as well as pattern genes like gray, tobiano, frame overo, and roan. Each follows the same two-letter format.
Cream is a common one. A single copy (N/Cr) on a bay base produces buckskin, on a chestnut base produces palomino, and on black produces smoky black. Two copies (Cr/Cr) create the more dramatic “double dilute” colors like cremello or perlino. The “N” stands for normal (no dilution allele), so N/N means the horse carries no cream gene at all.
Gray works similarly but is dominant: a single copy (G/N or G/G) means the horse will progressively gray out over its lifetime. Tobiano (T/N) produces the familiar large white patches over the body. Frame overo (O/N) creates irregular white patches along the sides, but two copies (O/O) is lethal in foals, which is why testing matters before breeding two overo-patterned horses together.
Reading Genetic Disease Results
Disease panels are the most straightforward section to interpret. Results use “N” for normal and a disease-specific abbreviation for the mutation. The three possible outcomes follow the same pattern across every disease:
- N/N: Normal. No copies of the disease allele. The horse is clear and cannot pass the mutation to offspring.
- N/[disease code]: Carrier. One copy of the mutation. For recessive diseases, the horse will not show symptoms but can pass the allele to 50% of its offspring.
- [disease code]/[disease code]: Affected. Two copies. For recessive conditions, the horse will develop the disease.
The Quarter Horse 5-panel test, which is one of the most commonly ordered panels, screens for five conditions: glycogen branching enzyme deficiency (GBED), hereditary equine regional dermal asthenia (HERDA), hyperkalemic periodic paralysis (HYPP), immune-mediated myositis (MYHM), and malignant hyperthermia (MH). Some labs bundle polysaccharide storage myopathy type 1 (PSSM1) as an additional test.
For example, a HERDA result of N/HRD means the horse carries one copy and is a carrier. It won’t develop the skin condition itself, but breeding it to another carrier gives each foal a 25% chance of being affected. A result of HRD/HRD means the horse has two copies and will develop the disease.
HYPP is a notable exception to the typical recessive pattern. It’s a dominant condition, so even a single copy (N/H) can cause episodes of muscle trembling and weakness. Horses that test positive for even one copy benefit from a low-potassium diet, and most breeders avoid using them for reproduction.
Parentage Verification Reports
Parentage results look quite different from color or disease panels. Instead of single-gene results, you’ll see a table of microsatellite markers, each with two numbers or letter codes representing the allele sizes your horse carries. In horses, labs typically report 15 standard markers, including AHT4, AHT5, ASB2, ASB17, ASB23, HMS2, HMS3, HMS6, HMS7, HTG4, HTG10, LEX3, LEX33, and VHL20, plus an amelogenin marker (AME) that confirms sex (reported as YX for males and X for females).
The principle is simple: at every marker, an offspring must share one allele with its dam and one with its sire. If a candidate parent doesn’t share an allele at even a single marker, that parent is excluded. You don’t need to interpret each number yourself. The lab’s conclusion will state whether each candidate parent “qualifies” or is “excluded.” What the raw data lets you do is compare profiles if you’re cross-referencing with registry records or resolving a disputed parentage case.
Breed Ancestry Results
Ancestry testing for horses works differently than the percentage-based reports you might be familiar with from dog DNA kits. Texas A&M’s ancestry test, one of the most widely used, compares your horse’s DNA profile against a reference panel of 50 breeds and reports the three breeds with the highest probability of being ancestral. These results are reported as ranked probabilities, not percentages of breed composition.
This is an important distinction. The lab is not saying your horse is 40% Thoroughbred and 30% Quarter Horse. It’s saying that, based on the genetic markers tested, the Thoroughbred reference population is the most likely source population. If your horse is a purebred, the correct breed will almost always appear as the top assignment. For a two-breed cross, both parental breeds typically rank high. But the more breeds involved in a horse’s background, the less reliable the assignments become.
Closely related breeds can also confuse the results. A horse that’s half Belgian Draft and half Suffolk Punch might show Percheron in its top results, because those heavy draft breeds look very similar at the genetic marker level. The top three breed assignments should be treated as a starting point for understanding heritage, not a definitive breakdown.
Using Results for Breeding Decisions
The real power of DNA results comes from combining your horse’s genotypes with a potential mate’s. For coat color, write out both parents’ genotypes at each locus and use a simple grid (called a Punnett square) to see the possible combinations. Two bay horses that are each heterozygous at both Red Factor and Agouti (E/e, A/a) can produce bay, black, or chestnut foals. But if even one parent is homozygous E/E, chestnut is off the table.
For disease testing, the critical question is whether both parents carry the same recessive mutation. Two N/N horses produce only clear offspring. One carrier bred to a clear horse produces no affected foals, though half the offspring will be carriers themselves. Two carriers give each foal a 25% chance of being affected. Running the math before breeding is straightforward once you understand the notation, and it’s the primary reason most owners order these tests in the first place.