Specific genetic test for the chestnut coat colour (reddish brown), applicable to all equine breeds. Chestnut occurs when the horse is homozygous for the recessive allele e of the MC1R gene (extension locus), which prevents the production of eumelanin in the hair and leaves only red pigment (phaeomelanin). The test identifies chestnut animals and, above all, hidden E/e carriers that are phenotypically bay or black. It is a direct tool for planning coat-colour matings.

The e allele is present in practically all equine populations, with highly variable frequencies by breed. In some it is fixed (the whole breed is chestnut, such as the Haflinger or the Suffolk Punch), whereas in others it is minoritary, generating frequent hidden E/e carriers.

- No clinical signs: a phenotype variant\n- Uniform reddish coat due to absence of eumelanin in the hair\n- Variable shades of red (light, ordinary, liver chestnut) due to modifier genes and environment\n- Mane and tail the same reddish tone as the body, except for white markings from other loci

The recessive nature of chestnut was known from classical genetics since the early 20th century, based on experimental crosses. In 1996, Marklund and colleagues showed that the e allele corresponds to a missense mutation (Ser83Phe) in the melanocortin 1 receptor (MC1R). This finding made it possible to develop the first DNA test for a coat colour in the horse. Genotyping spread quickly among breeders' associations, especially in breeds where chestnut is a desired or undesirable coat.

- Chestnut with chestnut always produces chestnut offspring (100% e/e)\n- Carrier E/e with chestnut results in 50% chestnuts and 50% non-chestnut carriers\n- Two E/e carriers mated together can produce chestnut (25%) even if both parents are bay or black\n- In an e/e the A locus is hidden: it cannot be inferred from the coat whether it carries bay or black\n- If your goal is to eliminate or fix chestnut, genotype the dark-coated breeding animals first

Do not confuse chestnut lightened by the sun or by modifier genes with genuine dilutions: the palomino, for example, is a chestnut with one cream allele. In breed registries with regulated coat colours, genotyping resolves doubtful coat parentages. Remember that the MC1R result does not inform about other panel loci unless they are tested separately.

1. Marklund L, Moller MJ, Sandberg K, Andersson L. A missense mutation in the gene for melanocyte-stimulating hormone receptor (MC1R) is associated with the chestnut coat color in horses. Mamm Genome 7(12):895-899, 1996. PMID: 8995760
2. Rieder S et al. Mutations in the agouti (ASIP), the extension (MC1R), and the brown (TYRP1) loci and their association to coat color phenotypes in horses (Equus caballus). Mamm Genome 12(6):450-455, 2001. PMID: 11353392
3. Wagner HJ, Reissmann M. New polymorphism detected in the horse MC1R gene. Anim Genet 31(4):289-290, 2000. PMID: 11086549
4. OMIA:001199-9796. Coat colour, extension in Equus caballus. https://omia.org/OMIA001199/9796/