Agouti: why bay and black look different
The ASIP gene and its A and a versions
A bay has a brown body with black mane, tail and lower legs. A black horse has black pigment over its body too. Agouti helps explain that distribution.
Explanations and diagrams reviewed on 3 November 2026
Extension and Agouti: the combinations behind each base
Read each line from left to right: the pair at Extension (MC1R), then the pair at Agouti (ASIP). These lists show the combinations that can give each base, not their probabilities in a cross.
Look at where the black appears
A bay keeps much of its black pigment in its mane, tail and lower legs, leaving a red-brown body. A black horse has black body hair as well. Sun bleaching, clipping and winter hair can blur the distinction.
These descriptions concern the base color. Other genes can lighten it or add white, so a brownish patch alone does not establish the horse’s genetic makeup.
One gene, two versions to understand
ASIP is the scientific symbol for Agouti. A horse inherits one version, or allele, from each parent. A restricts black pigment mainly to the mane, tail and lower legs. One A is enough: A/A and A/a give a bay base if the horse can make black pigment.
In the a version, a short stretch of DNA is missing. This disrupts the gene’s function. With a/a, black pigment is no longer restricted in this way and can extend over the body.
Why Agouti can be hidden in chestnut
The hair must first be able to make black pigment for Agouti to affect its distribution. Extension, the MC1R gene, controls that ability. Its E version allows black pigment production; the possible pairs containing E are E/E, E/e and E/eᵃ.
A chestnut with e/e cannot make black hair pigment. It can have A/A, A/a or a/a without showing any difference, yet still pass those Agouti alleles to its foals. A variant hidden in a parent’s coat can therefore influence the base color of its offspring.
What you can predict
Two tested A/a parents give each foal a 25% chance of a/a. That foal is black-based only if it also receives E at Extension; otherwise its base is chestnut. These chances apply to each conception, not a guaranteed sequence of births.
For another example, suppose one parent is E/E at Extension and a/a at Agouti, while the other is e/e and A/A. Every foal is E/e and A/a, giving a bay base before other modifiers act. A standard Agouti test distinguishes A/A, A/a and a/a. It does not predict an exact bay shade, explain every seal-brown appearance or test all Agouti variants proposed in the past.
An example: what could this foal inherit?
ASIP
Both demonstration horses have the exact pair shown. These examples explain inheritance; they do not use the mare and stallion entered in the calculator.
MareA/a
StallionA/a
All possible pairs at each conception
- A/A25%
Two copies of A
- A/a50%
One A and one a
- a/a25%
Two copies of a
Where the explanations come from
Laboratory resources explain which variants are tested and how to read their results. Research papers describe the evidence, discoveries and remaining uncertainties. Each article links its explanations to the relevant sources. Published titles stay in their original language so you can find the exact work.
1 · Laboratory explanation
UC Davis VGL — Agouti (Bay/Black)2 · Research paper
Rieder et al. (2001) — Mutations in ASIP, MC1R and TYRP1 and their association to horse coat colour