Agouti

Description

The Agouti gene (ASIP) controls the distribution of black pigment in the horse’s coat. Black pigment can either be spread evenly over the whole body or restricted to the “points” (ear rims, lower legs, mane, and tail), producing a bay horse. The recessive form of the gene (a) is caused by an 11-nucleotide deletion. Only horses with two copies of the deletion (a/a) have black pigment distributed evenly over the body, producing a black horse.

Horses with one (A/a) or two (A/A) copies of the dominant allele restrict black pigment to the points, because they carry at least one intact copy of the gene. The deletion must be present on both copies for black pigment to be evenly distributed.

Agouti has no visible effect on horses that are homozygous for Red Factor (e/e). These horses cannot produce black pigment, so Agouti has nothing to distribute, whether the horse is a/a, A/a, or A/A.

Even though Agouti is not visible on red (chestnut) horses, they can still carry and pass it on, so a red horse can produce a bay foal. This is where DNA testing becomes important. Breeders may want to test chestnut horses to learn whether they carry Agouti, or test bay horses to learn whether they carry one (A/a) or two (A/A) copies, which reveals the colors they can produce.

A horse with two copies (A/A) will always pass the dominant allele to its offspring, while a horse with one copy (A/a) has a 50% chance of passing it on. Testing is also useful when it is unclear whether a horse is truly black or a very dark bay, since the effects of other genes can make Agouti hard to judge by eye.

This test does not determine whether a horse is homozygous for black. To determine that, test for Red/Black Factor.

Possible results

Breeds Affected

All breeds

Reference

Mutations in the agouti (ASIP), the extension (MC1R), and the brown (TYRP1) loci and their association to coat color phenotypes in horses (Equus caballus).

Related Tests

• Red/Black Factor — determines whether the horse’s base pigment is red or black
• Silver Dilution — dilutes black pigment and can make a dark bay hard to read
• Cream Dilution — palomino, buckskin, cremello and perlino

Testing base color? The Base Color Panel covers Agouti and Red/Black Factor together for $40.