Locus E (red factor)

Locus E is one of the basic loci that determine the coat color of a horse. The only locus that is epistatic to it is locus A, which determines the final distribution of black pigment. If a horse carries the dominant E allele (genotype E/E, E/e, or E/ea) of the MC1R gene, eumelanin (black pigment) is produced due to a high level of tyrosinase, an enzyme involved in melanin synthesis. Phenotypically, the horse has a black coat (black). Complete black coloration of the body requires the simultaneous presence of the a/a genotype. The phenotype is modified by the presence of the A allele (the horse has black coloration on the mane, tail, and lower parts of the legs) and the At allele (dilution of the black color to dark brown).

In the presence of a mutation in the MC1R gene that prevents the expression of eumelanin, the horse may have the genotype e/e or ea/ea. The mutation allows the production of pheomelanin, the pigment responsible for the chestnut coat color. In summary, genotypes E/E, E/e, or E/ea are responsible for black, dark brown, or brown coloration. In contrast, genotypes e/e or ea/ea are responsible for a variable range of chestnut colors, from dark to light chestnut. The chestnut coloration is also expressed when the e/e or ea/ea genotype occurs together with the a/a genotype.

Two mutations in the MC1R gene are known to result in the production of red pigment (pheomelanin). In most horse breeds, the most common mutation is c.248 C>T (allele e). The Belgian Draft Horse breed has its own specific mutation, c.250G>A (allele ea). The test includes analysis of both of these genetic variants.

The inheritance pattern of the chestnut coat color is autosomal recessive. Chestnut coloration therefore occurs in individuals that inherit the mutated gene from both parents. These individuals are designated e/e or ea/ea. Carriers of the mutated gene, designated E/e or E/ea, have the mutated gene from only one parent. They can pass the mutation on to their offspring, but it does not affect their phenotype.

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References:

Marklund, L., Moller, M.J., Sandberg, K. et al. A missense mutation in the gene for melanocyte-stimulating hormone receptor (MCIR) is associated with the chestnut coat color in horses. Mammalian Genome 7, 895–899 (1996). https://doi.org/10.1007/s003359900264

Wagner HJ, Reissmann M. New polymorphism detected in the horse MC1R gene. Anim Genet. 2000 Aug;31(4):289-90. doi: 10.1046/j.1365-2052.2000.00655.x. PMID: 11086549.