The Agouti gene or ASIP (Agouti Signalling Protein) determines whether horses will have a bay or black base coat color.

Alleles

The agouti gene has two possible alleles. Agouti only affects horses with E/E E/e for extension; since chestnut-based horses (e/e) aren't able to produce any black pigment, agouti has no impact on their color*.

The dominant A-allele restricts black pigment (if present) to the points of the horse, resulting in a bay base color.

The recessive a-allele allows black pigment (if present) to be uniformly distributed over the coat, which results in a black base coat.

Extension Agouti Color
E/E
A/A
 Bay base
E/e
A/A
 Bay base
E/E
A/a
 Bay base
E/e
A/a
 Bay base
E/E
a/a
 Black base
E/e
a/a
 Black 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.

Agouti

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 agouti gene codes for a protein called ASIP. This protein blocks the receptors so that MSH can't get to them. As a result, the receptors can't be activated, and the affected areas will produce red pigment.

The a-allele is a mutation that makes this ASIP protein non-functional, so it can no longer block the receptors. Horses that are a/a can produce black pigment all over the body (if E/E or E/e).

The wild-type A-allele produces a normal ASIP protein. In horses that are A/A or A/a, black pigment will be restricted to the points of the horse as a result.

Alleles Mutation Effect
A
 / Black pigment is restricted to the points
a
 11 bp deletion
Frameshift mutation		 Black pigment over the entire body

Colors

Black
Black is one of the base colours in horses, alongside chestnut and bay. It is defined by a fully black coat.
Equine ColorsHannelore Roodhooft
Bay
Bay is one of the base coat colours in horses, alongside chestnut and black. It is defined by a red coat with black points in a wide range of shades.
Equine ColorsHannelore Roodhooft

Read more:
Color genes | Extension gene

Back to base colors

Articles

  1. Rieder, S., Taourit, S., Mariat, D., Langlois, B., Guerin, G.; Mutations in the agouti (ASIP), the extension (MC1R), and the brown (TYRP1) loci and their association to coat color phenotypes in horses (Equus caballus); Mammalian Genome (2001) Doi: 10.1007/s003350020017

*e refers to both e and ea alleles for simplicity, as they both have the same effect