Brindle Horses: Mechanisms, Genetics, and Patterns

A brindle horse carries irregular vertical streaks running down its body and horizontally around its legs, concentrated on the neck, shoulders, and hindquarters, generally sparing the head [Wikipedia: Brindle]. The streaks may differ from the base coat in color, texture, or both; in horses carrying the heritable Brindle 1 mutation, the striped hair is distinctly less straight and more unruly than the surrounding coat, in addition to any color difference [Murgiano et al. 2016, G3]. Brindle is among the rarest coat patterns documented in the species [Wikipedia: Brindle], and most of the confusion about it follows from a single unchecked assumption: that it works the way brindle works in dogs.

It does not. Canine brindle is controlled by the K locus on chromosome 16, where the kbr allele produces alternating eumelanin and phaeomelanin zones across the coat [Kerns et al. 2007, Genetics]. Equine brindle has no K-locus equivalent. Three genetically distinct mechanisms are confirmed with peer-reviewed evidence in horses, none of which involves the K locus [Wikipedia: Brindle; Murgiano et al. 2016; Towers et al. 2013, PLOS ONE]. Which mechanism is in front of you determines whether the pattern is heritable, whether it carries any health implication, and what a breeding record should say about it.

Three confirmed mechanisms

Chimerism

Most documented brindle horses are chimeric: two separately fertilized embryos fused early in development, producing a single animal whose cells carry two distinct genotypes [Wikipedia: Brindle]. Where the two cell populations differ in coat color (one bay, one chestnut, for example) the boundary between them traces the paths along which pigment cells migrated during fetal development, producing the visual signature of vertical brindle stripes [Kathman, Equine Tapestry, 2024]. These developmental pathways were first described by dermatologist Alfred Blaschko around 1901 and are now called Blaschko’s lines [Kathman 2024].

Chimeric brindle is not heritable. The reproductive cells arise from one or the other of the two component genomes, not from a blend, so the foal inherits a single ordinary coat genotype with no trace of the pattern [Kathman 2024]. A chimera can be confirmed by finding two distinct genotypes from tissue samples taken at different sites, or by a parentage test that returns apparent mismatches (more than two alleles per locus), a result that signals two genomes where one was expected [OMIA:000393-9796, Tetragametic chimerism, Equus caballus]. A 2018 study of 21,097 Purebred Spanish horses found chimerism at roughly 0.011% prevalence and concluded it is not especially connected to infertility [Anaya et al. 2018, via ScienceDaily].

Brindle 1 (BR1): the heritable form

In 2016 a peer-reviewed study identified a heritable equine brindle: an intronic variant in the MBTPS2 gene (c.1437+4T>C; genomic position NC_009175.3:g.17286855T>C on EquCab3.0) on the X chromosome, confirmed in a family of American Quarter Horses [Murgiano, Waluk, Towers et al. 2016, G3: Genes|Genomes|Genetics, doi:10.1534/g3.116.032433; OMIA:002021-9796]. The variant disrupts splicing: roughly 20% of MBTPS2 transcripts in affected skin skip exon 10 and parts of exon 11, deleting 32 codons that encode parts of the protein’s luminal and transmembrane domains [Murgiano et al. 2016]. The variant co-segregated perfectly with the phenotype across 39 family members and was absent from 457 control horses spanning 17 breeds [Murgiano et al. 2016].

Inheritance is X-linked semidominant. Heterozygous mares (one copy of the mutation) display the characteristic striped coat with altered hair texture. Hemizygous males (one copy, no balancing X) show sparse mane and tail but no visible stripe pattern [Murgiano et al. 2016]. The MBTPS2 gene encodes a zinc metalloprotease involved in sterol homeostasis; mutations in its human orthologue cause three genodermatoses. The equine BR1 variant produces only coat and hair-texture change with no systemic pathology reported [Murgiano et al. 2016]. A commercial genetic test for BR1 is offered by the UC Davis Veterinary Genetics Laboratory; see what the BR1 result means for breeders [OMIA:002021-9796].

This is the central split most sources elide: chimeric brindle and BR1 brindle look alike on the coat and behave in opposite ways for breeding. A chimeric brindle mare routinely produces non-brindle foals; a BR1 heterozygous mare passes the variant to approximately half her daughters. A photograph cannot tell them apart. Laboratory testing can.

Incontinentia pigmenti: brindle with disease

A third X-linked cause produces brindle-like stripes in mares but is a distinct systemic disorder, not a coat pattern. Incontinentia pigmenti (IP) in horses results from a nonsense mutation in the IKBKG gene (c.184C>T; p.Arg62*), first documented in a family of Quarter Horses in 2013 [Towers et al. 2013, PLOS ONE, doi:10.1371/journal.pone.0081625; OMIA:001899-9796]. Affected heterozygous mares develop progressive skin lesions following Blaschko’s lines, along with dental and hoof abnormalities; hemizygous males are typically lethal in utero [OMIA:001899-9796]. IP was found in the same Quarter Horse family as the BR1 study; the distinguishing feature is the multi-system pathology absent in BR1 horses [Murgiano et al. 2016].

A horse with brindle-like stripes and concurrent hoof or dental anomalies warrants consideration of IP. A horse with the stripe pattern and no systemic signs warrants consideration of BR1 or chimerism. The visual overlap between the three is real; the clinical and genetic separation is clean once the relevant evidence is gathered.

What brindle is not

Several patterns produce a striped or mottled-looking horse that gets called brindle in the absence of a better word. Each is a separate thing.

Roan intermingles white and colored hairs evenly across the body while the head, mane, tail, and lower legs (the “points”) retain the base color [Wikipedia: Roan (horse); Wikidata Q1520693]. Roan is present at birth and does not progressively lighten with age, which distinguishes it from gray [Wikipedia: Roan (horse)]. The underlying locus maps to the KIT gene region on equine chromosome 3, but no definitive causal mutation has been identified [Everts et al. 2025, Animals (Basel) 15(12):1705]. Roan does not stripe; brindle does not frost. Holding a bay roan and a chimeric brindle side by side, the distinction is immediate.

Rabicano (also called white ticking) places white hairs at the flank-stifle junction and the base of the tail (the “skunk tail”) and may extend as faint ticking along the barrel ribs [Wikipedia: Rabicano; Wikidata Q2033416]. Rabicano occurs in breeds that carry no true roan gene, including the Arabian, where the registry formally calls it “roan” [Wikipedia: Rabicano]. Its genetic cause is unresolved; a 2022 UC Davis thesis identified a candidate haplotype on chromosome 28 surrounding KITLG but could not confirm the causal variant [Esdaile & Bellone, UC Davis eScholarship 2022]. No commercial genetic test exists. The diagnostic markers that separate rabicano from brindle are the location (flank and tail rather than distributed vertically across the trunk) and the character of the white hairs (individual ticked white hairs scattered into color, not dark streaks separating color zones).

Manchado is an extremely rare white-spotting pattern documented only in Argentina, appearing in Thoroughbred, Criollo, Polo Pony, Arabian, and Hackney horses [Wikipedia: Pinto horse, citing Sponenberg & Bellone, Equine Color Genetics, 4th ed. 2017]. The pattern presents as large crisp white areas with smooth round colored spots inside them; the head and legs typically remain dark, and a white tail is consistent [Wikipedia: Pinto horse]. The genetic cause is not confirmed; the leading hypothesis is a rare recessive allele, but the 2024 peer-reviewed review of white coat color in horses omits manchado entirely, confirming no causal gene has been published [McFadden et al. 2024, Animals (Basel)]. Manchado and brindle share only their rarity; one is patchwork spotting with interior colored islands, the other is vertical striping without discrete spots.

Somatic mosaicism (distinct from chimerism) results when a single embryo’s cell acquires a mutation during development; every cell descended from it carries the change, and the resulting marked region again follows Blaschko’s lines [Wikipedia: Mosaic (genetics); Wikidata Q755077]. A chimera begins as two embryos; a mosaic begins as one. Both produce coat patterning along Blaschko’s lines and both are generally non-heritable (somatic, not germline). The distinction requires molecular testing to establish; the practical breeding implication is the same: neither form reliably reproduces the pattern [Kathman 2024].

Open questions

The genetic basis of brindle in horses is only partially resolved. Three mechanisms are confirmed with peer-reviewed evidence: tetragametic chimerism [OMIA:000393-9796], heritable BR1 (MBTPS2 variant) [Murgiano et al. 2016], and IP (IKBKG variant) [Towers et al. 2013]. Additional brindle cases exist that have not been assigned to any of these three, and whether further heritable loci exist beyond BR1 is an open research question [Wikipedia: Brindle]. The BR1 study was conducted in a single Quarter Horse / Paint Horse family of 39 animals; breed prevalence outside that lineage is not established in the published literature [Murgiano et al. 2016]. The precise mechanistic boundary between non-IP, non-BR1 Blaschko-line pigmentation and the other two mechanisms has not been crisply drawn in the retrieved literature. These uncertainties are not defects in the record; they are the frontier of what has been tested.

The 1997 archive

This domain has catalogued brindle and unusual-coat horses since 1997, before the MBTPS2 and IKBKG variants were characterized and before chimerism in horses had been confirmed by DNA testing. The 1997 brindle horse archive is a primary record of documented individual horses, assembled when mainstream genetics still treated these patterns as curiosities rather than subjects of genetic inquiry. It is the historical evidentiary floor under the claims on this page, and it predates the sources that those claims cite. It is kept intact as a dated primary source, not revised or modernized: a stable artifact from a specific moment in the field’s understanding, which is exactly what makes it useful as a baseline.

References

• Murgiano L, Waluk DP, Towers R, et al. An Intronic MBTPS2 Variant Results in a Splicing Defect in Horses with Brindle Coat Texture. G3: Genes|Genomes|Genetics. 2016;6(9):2963–2970. doi:10.1534/g3.116.032433. PMC5015953
• Towers RE, Murgiano L, Millar DS, et al. A Nonsense Mutation in the IKBKG Gene in Mares with Incontinentia Pigmenti. PLOS ONE. 2013;8(12):e81625. doi:10.1371/journal.pone.0081625. Full text
• Kerns JA, Cargill EJ, Clark LA, et al. Linkage and Segregation Analysis of Black and Brindle Coat Color in Domestic Dogs. Genetics. 2007;176(3):1679–1689. PMC1931550
• Everts RE, et al. Novel Equine Roan Haplotypes and Prevalence of the RN1 and RN2 Haplotypes in Multiple Breeds. Animals (Basel). 2025;15(12):1705. PMC12189688
• McFadden A, et al. Spotting the Pattern: A Review on White Coat Color in the Domestic Horse. Animals (Basel). 2024. PMC10854722
• Anaya G, Fernandez ME, Valera M, et al. Prevalence of twin foaling and blood chimaerism in purebred Spanish horses. Vet J. 2018. Open summary via ScienceDaily
• Esdaile ES; advisor Bellone RR. Short Tandem Repeat Analysis of Genetic Diversity Metrics in American Standardbreds and an Investigation on the Cause of the Rabicano Coat Color Phenotype. UC Davis eScholarship, 2022. Full text
• Kathman L. Mosaicism in Horses – Part 1. Equine Tapestry. 2024-05-09. equinetapestry.com
• OMIA:002021-9796 – Brindle 1, Equus caballus. omia.org (last updated 2026-05-31)
• OMIA:001899-9796 – Incontinentia pigmenti, Equus caballus. omia.org
• OMIA:000393-9796 – Tetragametic chimerism (including Freemartin), Equus caballus. omia.org
• Wikipedia: Brindle. en.wikipedia.org/wiki/Brindle (Wikidata Q1969557)
• Wikipedia: Roan (horse). en.wikipedia.org/wiki/Roan_(horse) (Wikidata Q1520693)
• Wikipedia: Rabicano. en.wikipedia.org/wiki/Rabicano (Wikidata Q2033416)
• Wikipedia: Mosaic (genetics). en.wikipedia.org/wiki/Mosaic_(genetics) (Wikidata Q755077)
• Sponenberg DP, Bellone R. Equine Color Genetics, 4th ed. Wiley Blackwell, 2017. [Cited via Wikipedia Pinto horse article for manchado description]

For a comparison with a well-characterized spotting pattern that is definitively not brindle, the appaloosa LP complex (TRPM1) and the tobiano KIT inversion are covered separately. Manchado and pinto are both white-spotting categories that registries handle differently from brindle’s stripe-based patterns. The pinto grouping (which covers overo, tobiano, tovero, and several rarer pattern subtypes) is documented at horse-info.org’s pinto entry. For owners managing a horse whose striped or unusual coat raises questions about ongoing coat health, sickhorses.com’s article on hair loss in horses covers the dermatological and nutritional causes of coat change that can complicate visual assessment of a genetically patterned animal.