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Horse Genetic Testing for Breeding: PubMed Evidence Review

November 03, 2026

Reviewed

Horse Genetic Testing for Breeding: PubMed Evidence on Disease, Colour, Performance and Inbreeding

A known causal DNA variant exists for fewer than 20% of suspected inherited traits in horses (PMID: 38600096). Where tests do exist, breed averages can hide high risk in specific lines: among sampled Quarter Horse halter horses, 56.4% could pass on the HYPP variant, while the whole-breed allele frequency was 0.008 (PMID: 19119976). This review sets out what the published literature reports on inherited disease, coat colour, performance, inbreeding and complex disease in horses.

This post is derived from a verified BioSkepsis research thread View the research on BioSkepsis →

Inherited equine diseases: causal mutations, carrier rates and mating risk

A carrier is a horse with one copy of a disease variant. For recessive diseases, which need two copies to cause illness, a mating of two carriers produces an affected foal 25% of the time. For dominant diseases, which need only one copy, a single carrier parent passes the variant to 50% of foals (PMID: 19119976).

The same study notes that artificial insemination, superovulation and cloning let a popular carrier sire spread a variant widely within a few years. AQHA rules from 2007 excluded horses with two copies of the HYPP variant (H/H) from registration (PMID: 19119976).

Allele frequency in American Quarter Horses: whole breed versus the highest-risk performance group. Allele frequency is the share of all gene copies in the population that carry the variant.
Disease (gene) Whole breed Highest subgroup
HYPP (SCN4A) 0.008 0.299 in halter horses; 56.4% able to pass it on
PSSM1 (GYS1) 0.055 0.155 in halter horses; 28.2% carriers
HERDA (PPIB) 0.021 0.142 in cutting horses; 28.3% carriers
GBED (GBE1) 0.054 0.132 in western pleasure horses
Immune-mediated myositis (MYH1 E321G) 0.034 0.135 in reining horses

Sources: HYPP, PSSM1, HERDA and GBED rows (PMID: 19119976); MYH1 row (PMID: 30623495), a variant linked to immune-mediated myositis, a muscle inflammation disorder (PMID: 29510741). Racing and barrel racing groups were largely free of these variants (PMID: 19119976).

HYPP (hyperkalaemic periodic paralysis, a muscle disorder) is linked to the muscle sodium channel gene SCN4A (PMID: 1323940). The variant spread through the breed from a founder sire through selective breeding (PMID: 1338908).

HERDA (hereditary equine regional dermal asthenia, a fragile skin disease) is caused by the PPIB c.115G>A variant (PMID: 17498917). In 1,295 Quarter Horses, 58 of them affected, heritability was 0.38 ± 0.13. Estimated carrier frequency was 1.84% among mares bred to carrier stallions overall but 6.45% within one popular stallion's line, and affected horses had about four times the mean inbreeding of controls (PMID: 15822588).

GBED (glycogen branching enzyme deficiency, a fatal sugar storage disease of foals) is caused by a GBE1 variant. Allele frequency was 0.041 in Quarter Horses (8.3% carriers), 0.036 in Paints (7.1%) and 0.000 in Thoroughbreds. Foals with two copies made up about 2.5% of fetal and early newborn deaths of Quarter Horse lineage at two diagnostic laboratories, mostly as abortions (PMID: 17063718).

PSSM1 (type 1 polysaccharide storage myopathy) is caused by a GYS1 variant found in 17 breeds. Among horses diagnosed with PSSM, the variant was present in 87% of Draft and 72% of Quarter Horse-related breeds, against 18% of Warmbloods (PMID: 18691366).

Warmblood fragile foal syndrome (WFFS, PLOD1 c.2032G>A)

Screening of 4,081 horses in 38 breeds found 4.9% carriers overall, about 17% in Hanoverian and Danish Warmbloods, about 11% across Warmbloods and 2.37% in Thoroughbreds (PMID: 33353040). In Hanoverians, most carriers trace to one stallion born in 1861 (PMID: 32323341). In US Thoroughbreds, 2.4% are carriers and the variant is not linked to catastrophic breakdown injury (PMID: 31502696). Of 14 foals with two copies, most were born alive but never tried to stand, and 12 had visible skin defects (PMID: 31785623).

Other lethal recessive diseases with carrier data

Malignant hyperthermia (RYR1 c.7360C>G, a reaction often set off by anaesthesia) was found only in Quarter Horse-related breeds, at allele frequencies of 0.0013 in Quarter Horses and 0.0012 in Paints and Appaloosas, with no horse carrying two copies among 159,227 tested; it was fatal in 51% of observed carriers (PMID: 40298507). The lavender foal syndrome MYO5A deletion was found in 10.3% of sampled Egyptian Arabians (PMID: 20419149). Tests also exist for overo lethal white syndrome (EDNRB) in Paint Horses (PMID: 9585428; PMID: 30096357) and foal immunodeficiency syndrome in Fell and Dales ponies (PMID: 21750681).

Horse coat colour genetics: what DNA tests predict and where they fail

Base coat colour depends on a small, well-tested set of genes. Dilution colours and white patterns add further genes, some of which interact.

Coat colour traits and the genes reported in the literature
Coat trait Gene What the literature reports
Chestnut MC1R A single-letter change in the gene is linked to chestnut (PMID: 8995760)
Black, bay, brown MC1R with ASIP The two genes act together to set these colours (PMID: 30785190)
Cream, pearl, sunshine SLC45A2 Complete match between genotype and colour in testing (PMID: 31006892)
Champagne SLC36A1 Variant in exon 2 (PMID: 18802473)
Dun TBX3 In Konik horses the variant does not always show in the coat, and a further allele changes the degree of dilution (PMID: 34432873)
Leopard complex spotting TRPM1 A 1,378 bp viral-origin insertion causes both the pattern and night blindness (PMID: 24167615)
Splashed-white-like pattern MITF A deletion of about 63 kb comes with a higher risk of deafness (PMID: 30644113)
Curly coat KRT25 and SP6 The KRT25 variant masks the effect of the SP6 variant (PMID: 29686323)
Grey Locus on chromosome 25 Mapped to equine chromosome 25 (PMID: 12354140)

Several of these genes can be tested together in one assay (PMID: 19912416). Prediction is weaker for white patterns. In 1,195 American Paint Horses, the share of white coat rose with each spotting allele carried, whatever the gene, and the best model explained at best about 55% of the variation (PMID: 38002953).

Registry records are a further source of error. In the Sarcidano horse, genetic and recorded colour disagreed in 53.4% of cases, with many horses recorded as bay being genetically black (PMID: 38396512). In Hucul horses, TBX3 and ASIP tests disagreed with official colour records in about a third of animals (PMID: 30614426).

A survey of 605 horse genomes found that the SLC45A2 sunshine variant, reported as a rare dilution, is the main allele (above 0.86) in Arabians, Belgians, Morgans, Thoroughbreds and Quarter Horses. Several KIT white-marking and roan variants were also common across breeds, at levels that do not fit their reported effects (PMID: 38600096).

Myostatin (MSTN) and race distance: how far equine performance genetics predicts

Myostatin is a protein that limits muscle growth. The MSTN gene that makes it holds the best-established performance marker in horses.

MSTN g.66493737C>T in Thoroughbreds

The marker was strongly linked to best race distance in elite race winners (P = 4.85 × 10-8). The C allele was about twice as common in short-distance as in long-distance horses (0.72 against 0.36). Mean best race distance was 6.2 furlongs for C/C, 9.1 for C/T and 10.5 for T/T horses; a furlong is about 201 metres (PMID: 20098749).

A genome-wide study confirmed the same region, with its top marker closely tied to the MSTN marker (r2 = 0.86) (PMID: 20932346). Laboratory work points to a 227 bp SINE insertion, a short piece of mobile DNA in the gene's control region, as the working variant: it cut myostatin production about 4.5-fold, while the original marker had no measurable effect (PMID: 30379863). Horses with two copies of the insertion had lower blood myostatin (PMID: 33846367).

The marker does not work the same way in every breed. The insertion is common only in Thoroughbreds (0.48) and Quarter Horses (0.73). In Belgian draft horses, which lack it, the original marker showed no link with muscle fibre type (PMID: 25160752). In Anglo-Arabians, also without the insertion, horses with one copy of each allele raced better, which the authors tentatively attribute to a hybrid advantage (PMID: 33808485). In 1,710 Japanese Thoroughbreds, the effects differed by sex (PMID: 22221024).

Other genes also matter. A study of 1,156 Thoroughbreds found 28 linked markers in 17 genes, many involved in muscle building (PMID: 25925054). In the Norwegian-Swedish Coldblooded Trotter, linked genes concerned learning and energy use instead (PMID: 30157760). Three fracture-risk variants near MSTN are the main alleles in Thoroughbreds (51.3% to 55.3%) and occur in all breed groups, so any effect on fracture risk is likely small (PMID: 38600096).

Genetic diversity and inbreeding in horse breeds: genomic methods and reported levels

Most modern studies use DNA chips that read tens of thousands of markers at once, starting with the 54,602-marker EquineSNP50 chip (PMID: 22253606). Three types of measure are compared: inbreeding from pedigree records (Fped), genomic measures of excess matching DNA, and FROH. FROH is the share of the genome in runs of homozygosity, meaning long stretches where both inherited copies are identical; longer runs point to more recent inbreeding.

Reported inbreeding levels in selected horse populations
Population Reported value Source
Thoroughbred Inbreeding coefficient about 0.15, the highest among breeds studied on the chip PMID: 22253606
Sable Island feral horses Mean FROH 0.29, up to 0.42; only about 4% of the genome in runs longer than 16 Mb PMID: 35820826
Sicilian Arab FROH 0.19 PMID: 35445758
Purosangue Orientale Siciliano FROH 0.13; current effective population size 10 PMID: 35445758
Siciliano FROH 0.07 PMID: 35445758
Rhenish German Draught Horse FROH 0.099 against Fped 0.016; recent effective population size about 59 PMID: 40149478

Effective population size is the number of breeding animals that would produce the observed loss of diversity; low values signal risk. For Sable Island horses, the small share of very long runs indicates inbreeding mainly from past population crashes rather than recent close matings (PMID: 35820826). In the Rhenish German Draught Horse, the gap between FROH and Fped is attributed to short pedigree records (PMID: 40149478).

Inbreeding also links to fitness. In Pura Raza Espanola mares, higher whole-genome homozygosity went with lower fertility breeding values, and 13 of 41 homozygosity hotspots were linked to fertility (PMID: 35252415). Across 605 horses, the median number of predicted harmful variants per horse was 730 (IQR 613 to 829), ranging by breed from 607 in Thoroughbreds to 860 in Icelandic horses (PMID: 38600096).

Complex equine diseases: heritability of metabolic, laminitis, tendon, bone and colic risk

Complex diseases are shaped by many genes plus environment. Heritability is the share of variation in a trait explained by genetic differences, on a scale from 0 to 1.

Selected heritability and genetic findings for complex horse diseases
Condition Population Reported finding
Equine metabolic syndrome traits Welsh ponies, Morgan horses Moderate to high heritability for most of nine traits, including insulin (PMID: 30472742)
Pasture-associated laminitis Ponies Pattern fits a dominant major gene that does not always show its effect (PMID: 16677122)
Distal limb fracture UK Thoroughbreds 0.21 to 0.37 (PMID: 24679457)
Superficial digital flexor tendon injury UK Thoroughbreds 0.31 to 0.34; genetic correlation with fracture 0.44 (PMID: 24679457)
Fracture, osteoarthritis, ligament and tendon injury Hong Kong Thoroughbreds About 0.01 to 0.20, described as preliminary (PMID: 23746478)
Degenerative suspensory ligament desmitis Peruvian Horse 0.22 ± 0.08; polygenic risk score R2 about 0.70, not yet validated (PMID: 37645058)
Large colon volvulus (a colic cause) Kentucky Thoroughbred broodmares Moderate in a preliminary estimate with a wide margin of error (PMID: 31391286)

Metabolic disease. A risk region near FAM174A was found in Arabians (PMID: 28380523) but showed no link with obesity or insulin measures in a small pony group (PMID: 32169753). A combined blood-chemistry and genome study in Arabians found four candidate regions holding 63 genes, with top markers explaining 43.52% of variation in a combined syndrome score (PMID: 32659253). In Welsh ponies, the HMGA2 height variant also tracks insulin and blood fats, explaining about 40% of height heritability and about 20% of insulin heritability (PMID: 30666754).

Other conditions. Osteochondrosis risk regions on chromosome 14 were found in US Standardbreds and confirmed in Norwegian Standardbreds (PMID: 26753841). Sarcoid tumour risk maps to an immune gene region (MHC class II DQA1), with an odds ratio of 5.05, plus a region on chromosome 22 with an odds ratio of 4.02 (PMID: 27037728). Grass sickness clustered in families in one Hungarian stud, with no gene identified (PMID: 33128520). The DDB2 T338M variant is a recessive risk factor for eye squamous cell carcinoma in several breeds (PMID: 33291392; PMID: 31637255).

Gaps in equine genetic testing: misclassified variants and unvalidated commercial tests

Healthy horses carry many predicted harmful variants: a median of 730 per horse, about 1.4 to 2.6 times human estimates. A variant found in many unrelated healthy horses is therefore unlikely to cause a disease on its own (PMID: 38600096).

The same 605-horse study lists concrete cases. The five reported congenital liver fibrosis variants occur in more than nine breeds, above 89% frequency in some, and 369 of 605 horses carried at least one. The juvenile idiopathic epilepsy variant is the main allele in Standardbreds and Arabians (PMID: 38600096).

PSSM2 and myofibrillar myopathy tests

All six commercially marketed PSSM2 variants occur in several breeds at frequencies above 5% (PMID: 38600096). An independent study found that commercial tests (P2, P3a/b, P4) showed no significant link with a diagnosis made by muscle tissue examination in Warmbloods or Arabians, and the variants were also present in ancient domestic horses (PMID: 32896939).

Other failures to replicate include the FAM174A metabolic syndrome variant in ponies (PMID: 32169753) and the KCNE4 anhidrosis variant, which did not predict the condition in a well-described herd (PMID: 39953936). The horse reference genome comes from a Thoroughbred, which can inflate apparent variant counts in distant breeds and hide Thoroughbred-specific variants (PMID: 38600096).

Applying equine genetic test evidence: breeders, veterinarians and registries

Test the sire lineQuarter Horse and Warmblood breeders

The literature supports mate selection based on test results for the specific sire lines involved, not breed averages (PMID: 19119976). Carrier rates in subgroups can exceed whole-breed rates by an order of magnitude (PMID: 33353040).

Base coloursColour breeders and breed registries

Colour tests are reliable for base colours but less reliable for white patterns and dilution combinations, and visual registry records introduce error (PMID: 38396512; PMID: 38002953).

Breed-specificRacehorse breeding programmes

MSTN is informative for distance aptitude in Thoroughbreds, but its working variant and its limits in other breeds are now described (PMID: 30379863; PMID: 25160752). The rest of performance variation involves many genes that differ by breed (PMID: 25925054).

Check validationEquine veterinarians ordering tests

The literature calls for marketed variants to be re-checked against population genome data before new tests are sold (PMID: 38600096; PMID: 32896939).

Frequently asked questions about equine genetic testing

Which inherited horse diseases do breeders test for?

The published literature reports DNA tests for HYPP, HERDA, GBED, PSSM1 and MYH1-related myositis in Quarter Horse-related breeds; WFFS in Warmbloods; overo lethal white syndrome in Paint Horses; cerebellar abiotrophy, severe combined immunodeficiency and lavender foal syndrome in Arabians; foal immunodeficiency syndrome in Fell and Dales ponies; and malignant hyperthermia (RYR1) in Quarter Horse-related breeds.

How common are HERDA, HYPP and GBED carriers in Quarter Horses?

Across the whole breed, allele frequencies were 0.021 for HERDA, 0.008 for HYPP and 0.054 for GBED (PMID: 19119976). Allele frequency is the share of all gene copies in the population that carry the variant. In specific performance groups the values were several times higher: HYPP reached 0.299 in halter horses, HERDA 0.142 in cutting horses and GBED 0.132 in western pleasure horses.

Can a DNA test predict a foal's coat colour?

Prediction is reported as accurate for simple base colours such as chestnut, bay and black, which depend mainly on MC1R and ASIP. It is less accurate for white patterns: in 1,195 American Paint Horses, the best model explained at best about 55% of the variation in white coat (PMID: 38002953).

Does the myostatin (MSTN) speed gene test predict race distance in every breed?

No. In Thoroughbreds the MSTN marker is strongly linked to best race distance (PMID: 20098749), and a 227 bp SINE insertion, a short piece of mobile DNA in the gene's control region, is the likely working variant (PMID: 30379863). In Belgian draft horses, which lack the insertion, the marker showed no link with muscle fibre type (PMID: 25160752).

How is inbreeding measured in horses?

Studies compare pedigree-based inbreeding (Fped) with genomic measures, mainly FROH, which is the share of the genome found in runs of homozygosity, meaning long stretches where both inherited copies are identical. In the Rhenish German Draught Horse, FROH averaged 0.099 while Fped was only 0.016, with limited pedigree depth the likely reason (PMID: 40149478).

Are all commercial equine genetic tests validated?

No. Commercial PSSM2 and myofibrillar myopathy tests showed no significant link with a diagnosis made by muscle tissue examination in Warmbloods or Arabians (PMID: 32896939). A 605-horse genome study found all six marketed PSSM2 variants in several breeds at frequencies above 5% (PMID: 38600096).

Which complex horse diseases have a genetic component?

Heritability, the share of variation in a trait explained by genetic differences, has been measured for equine metabolic syndrome traits, limb fracture, tendon injury, suspensory ligament desmitis and several eye conditions. Causal variants for these diseases remain largely unidentified.

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Sources & further reading: PubMed records

  1. PMID 38600096: Genome-wide prediction of genetic burden and phenotype-associated variant frequencies in 605 horses.
  2. PMID 19119976: Allele frequencies of inherited disease genes in American Quarter Horse subgroups.
  3. PMID 30623495: Prevalence of the MYH1 E321G variant in American Quarter Horse performance subgroups.
  4. PMID 29510741: MYH1 E321G and immune-mediated myositis in Quarter Horses.
  5. PMID 1323940: HYPP linked to the equine skeletal muscle sodium channel gene.
  6. PMID 1338908: Spread of the periodic paralysis sodium channel mutation by selective breeding.
  7. PMID 17498917: PPIB missense mutation in HERDA.
  8. PMID 15822588: Inheritance analysis of HERDA in Quarter Horses.
  9. PMID 17063718: GBE1 allele frequency in Quarter Horse and Paint Horse populations.
  10. PMID 18691366: GYS1 mutation across diverse breeds with PSSM.
  11. PMID 33353040: WFFS distribution across breeds in Europe and the United States.
  12. PMID 32323341: Hanoverian F/W line and WFFS segregation.
  13. PMID 31502696: WFFS in Thoroughbreds and catastrophic breakdown.
  14. PMID 31785623: Foals, abortions and stillbirths homozygous for WFFS.
  15. PMID 40298507: RYR1 malignant hyperthermia allele frequencies and genotypes.
  16. PMID 20419149: MYO5A deletion causing lavender foal syndrome.
  17. PMID 9585428: EDNRB mutation in lethal white foal syndrome.
  18. PMID 30096357: High-resolution melting test for EDNRB in Paint Horses.
  19. PMID 21750681: Foal immunodeficiency syndrome mutation in Fell and Dales ponies.
  20. PMID 8995760: MC1R missense mutation and chestnut coat colour.
  21. PMID 30785190: MC1R and ASIP interaction in horse coat colour.
  22. PMID 31006892: SLC45A2 variants in pearl and sunshine dilutes.
  23. PMID 18802473: SLC36A1 and champagne dilution.
  24. PMID 34432873: Dun dilution in the Konik horse.
  25. PMID 24167615: TRPM1 retroviral insertion in leopard complex spotting and night blindness.
  26. PMID 30644113: MITF deletion, white spotting and deafness risk.
  27. PMID 29686323: KRT25 and SP6 in curly coat.
  28. PMID 12354140: Grey locus mapped to equine chromosome 25.
  29. PMID 19912416: Simultaneous genotyping of several coat colour loci.
  30. PMID 38002953: Digital phenotyping and white spotting interaction in American Paint Horses.
  31. PMID 38396512: Genetic versus visual coat colour in the Sarcidano horse.
  32. PMID 30614426: TBX3 and ASIP genotypes against recorded colours in Hucul horses.
  33. PMID 20098749: MSTN polymorphism, sprinting ability and stamina in Thoroughbreds.
  34. PMID 20932346: Genome-wide confirmation of the MSTN race distance marker.
  35. PMID 30379863: MSTN promoter SINE insertion as the speed gene variant.
  36. PMID 33846367: MSTN SINE insertion and blood myostatin levels.
  37. PMID 25160752: MSTN haplotype diversity, race distance and muscle fibre type across breeds.
  38. PMID 33808485: MSTN polymorphisms and performance in Anglo-Arabian racehorses.
  39. PMID 22221024: Chromosome 18 markers and racing in Japanese Thoroughbreds.
  40. PMID 25925054: Two-stage genome-wide study of racing performance in 1,156 Thoroughbreds.
  41. PMID 30157760: Genome-wide study of harness racing in the Norwegian-Swedish Coldblooded Trotter.
  42. PMID 22253606: High-density SNP array for the domestic horse.
  43. PMID 35820826: Runs of homozygosity in Sable Island feral horses.
  44. PMID 35445758: Runs of homozygosity in three Sicilian horse populations.
  45. PMID 40149478: Homozygosity and diversity in the Rhenish German Draught Horse.
  46. PMID 35252415: Homozygosity islands and fertility in mares.
  47. PMID 30472742: Heritability of metabolic syndrome traits in Welsh ponies and Morgan horses.
  48. PMID 16677122: Genetic and metabolic risk factors for pasture-associated laminitis in ponies.
  49. PMID 28380523: Metabolic syndrome risk region in Arabian horses.
  50. PMID 32169753: FAM174A and metabolic syndrome in ponies.
  51. PMID 32659253: Metabogenomic candidate regions for equine metabolic syndrome.
  52. PMID 30666754: HMGA2 effects on height and metabolic traits in ponies.
  53. PMID 24679457: Genetic parameters for limb fracture and tendon injury in UK Thoroughbreds.
  54. PMID 23746478: Preliminary genetic analysis of musculoskeletal conditions in Hong Kong Thoroughbreds.
  55. PMID 37645058: Genetic architecture and risk score for suspensory ligament desmitis in the Peruvian Horse.
  56. PMID 31391286: Preliminary heritability of large colon volvulus in Thoroughbred broodmares.
  57. PMID 26753841: Osteochondrosis risk regions in Standardbreds.
  58. PMID 27037728: Host genetics and papillomavirus-induced tumours in horses.
  59. PMID 33128520: Family clustering of equine grass sickness in a Hungarian stud.
  60. PMID 33291392: DDB2 risk factor for eye squamous cell carcinoma in three more breeds.
  61. PMID 31637255: Further evidence for DDB2 T338M in eye squamous cell carcinoma.
  62. PMID 32896939: Commercial PSSM2 and myofibrillar myopathy tests against tissue diagnosis.
  63. PMID 39953936: KCNE4 variant not predictive of equine anhidrosis.
  64. Source research thread: BioSkepsis research on genomic and genetic testing in horses.