The extension or MCR1 (melanocortin 1 receptor) gene, also sometimes called "black/ red factor", determines whether a horse will have a chestnut or a black/bay base coat color.

Alleles

The extension gene has three possible alleles.

The dominant E-allele allows for the production of black pigment and leads to a black or bay base color depending on the agouti gene.

The recessive alleles e and ea make horses unable to produce black pigment. The result is red pigment only. e and ea have the same effect and can't be distinguished visually.. The ea ("alternate") allele is rare and has only been found in a few horse breeds.

Extension Color
E/E
 Black or bay base, depending on agouti
E/e
 Black or bay base, depending on agouti
e/e
 Chestnut base
e/ea
 Chestnut base
ea/ea
 Chestnut base

Mechanics

Pigment cells, called melanocytes, produce the pigments in the hair and skin of horses. The production of pigments can be influenced at different stages, changing the type (black or red) or the amount (dilutions/ white patterns) of pigment that is made.

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The production of black and red pigment involves several different steps. Some of these steps are shared between the two, and some are unique to each pigment type. This is why certain genes will affect all base colors, while others only affect either red or black pigment: they affect different steps.

Extension

Melanocytes can produce two types of pigment: eumelanin (black) and pheomelanin (red). Which pigment is made depends on a receptor on their surface, called MC1R. This receptor can be activated by a protein known as MSH.

  • When the receptor is activated by MSH, black pigment is formed.
  • When there is no activation by MSH, cells produce red pigment.

The extension gene codes for this MC1R receptor. Mutations in the gene can lead to a receptor that no longer works and can't be activated at all, with red pigment as a result.

There are two such mutations of the extension gene: the alleles e and ea. Horses that are e/e, eᵃ/eᵃ or e/eᵃ have no functional receptors and can produce only red pigment.

The wild-type E-allele leads to normal receptors. This makes it possible for horses that are E/E or E/e to produce black pigment.

Alleles Mutation Effect
E
 / Black pigment is possible
e
 Point mutation/ substitution (C => T)
missense mutation		 Red pigment
ea
 Point mutation/ substitution (G => A)
missense mutation		 Red pigment

Colors

Chestnut
Chestnut is one of the base coat colours in horses, next to black and bay. It is characterised by a fully red coat.
Equine ColorsHannelore Roodhooft
Black
Black is one of the base colours in horses, next to chestnut and bay. It is characterised by a fully black coat.
Equine ColorsHannelore Roodhooft
Bay
Bay is one of the base coat colours in horses, next to chestnut and black. It is characterised by a red coat with black points.
Equine ColorsHannelore Roodhooft

Read more:
Color genes | Agouti gene

Back to base colors

Articles

  1. Marklund, L., Moller, M. J., Sandberg, K., & Andersson, L.; A missense mutation in the gene for melanocyte-stimulating hormone receptor (MCIR) is associated with the chestnut coat color in horses, Mammalian Genome, 1996; 10.1007/s003359900264
  2. Wagner, H-J., & Reissmann, M.; New polymorphism detected in the horse MC1R gene; Animal Genetics, 2000; 10.1046/j.1365-2052.2000.00655.x
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