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Locus Information: E-Locus (Extension, MC1R gene)

Inheritance: autosomal recessive

→ Horses with two copies of this variant (ee) are chestnut (red, sorrel) in color. Horses with only one copy of this variant (Ee) are bay or black (depending upon their variants at the Agouti locus), but carry chestnut. A horse that does not carry the variant (EE) cannot have a chestnut foal, even if bred to a chestnut horse.

General Informationen:

  • The Extension locus determines if a pigment-producing cell can make both red (pheomelanin) and black (eumelanin) pigments.
  • If it is non-functional due to a genetic mutation, the cell can no longer produce black pigment.
  • When only red pigment is produced the horse will be chestnut.
  • Horses with one or two functional copies of the Extension gene (EE, Ee) are bay or black (depending upon the variants at their Agouti gene).

Possible Genotypes:

Genotype: Effect:
EE Two normal, functional copies → the horse is bay or black, depending on the A locus.
Ee One functional copy and one non-functional copy → the horse is bay or black, depending on the A locus.
ee Two non-functional copies → the horse has a chestnut coat color.


A horse’s base color is determined by the combination of its Agouti and Extension gene variants.

AA Aa aa



Carries factors for black





Carries factors for




Carries factors for

chestnut and black



Carries factors for



ee AAee



Carries factors for black


Carries factors for black


Test information: This test detects two mutations in the MC1R gene.

Marklund, L., Moller, M.J., 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. Mammalian Genome 7:895-899, 1996. Pubmed reference: 8995760.

Wagner, H.J., Reissmann, M.: New polymorphism detected in the horse MC1R gene. Anim Genet 31:289-90, 2000. Pubmed reference: 11086549.

More Information if available at the Online Mendelian Inheritance in Animals website.

Examples of horses with the chestnut base color, here with different white patterns. The genetic variants responsible for the different shades and color intensities are not yet known.

Test #: H203

Sample: Blood, Hair

Price: 35 € (net)

Time: 10-14 days

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