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Bay Horses: Agouti gene testing

The common Bay colour is seen across a wide-range of horse types and breeds. The dominant Agouti gene is responsible for the production of Bay Horses. In simple terms, a Bay horse has black pigment but is a carrier of the Agouti gene.

The Agouti factor suppresses the Black pigment of the horse to "Points" on the carrier's body, these are usually the lower legs, socks, mane, tail and ear-rims. This leaves the rest of the horse's body a Brown colour, known as Bay. There are many tones and shades of Bay, which varies from horse to horse.

When testing for the Agouti gene, there can be three potential outcomes. These are coded as follows:

aa: Non-Agouti Horse. Black pigment is uniformly distributed.

Aa: Agouti Heterozygous. Horse carries one copy of Agouti, basic colour of this horse will be Bay. 50% likelihood of passing on the Agouti gene to its offspring.

AA: Agouti Homozygous. Horse carries two copies of Agouti. Basic colour of this horse will be Bay. An Agouti Homozygous horse will always pass the Agouti gene on to its offspring.

Please note, the Agouti gene has NO effect on Red/Chestnut based horses. They can carry the gene, but as Agouti acts on Black pigment, it is not visibly determined. Since Bay is a type of Black, there are some Bay specimens which carry the recessive red (chestnut/sorrel) gene.

For those interested in breeding bay foals, Homozygous Agouti horses are ideal as they will always transmit the agouti gene when bred. Those looking to 'breed out' the Bay colouring to gain Black foals, may hope for Heterozygous Agouti horses.

Testing is also useful as some horses are a very dark Bay, which may need genetic identification from black.

If you are unsure which tests apply specifically to your horse,

click here.

Testing procedure & Frequently asked questions

Click here if you wish to test your horse for this trait.