Bay - The Agouti Gene

Now that we know the two base colors that make up all horse genetics, we can start looking at other colors. Many colors are caused by modifiers. Modifiers are alleles that change the appearance of the base color. One of the most well-known modifiers and most identified is the agouti gene, which creates BAY.


The above picture is of Proud Man's Cherokee Chief, aka Maverick, a Tennessee Walking Horse. Maverick is a excellent example of a bay horse. When the agouti allele is present, it acts on the black allele, restricting the coloration to the "points" of the horse. The points are the lower legs, ear tips, mane, and tail. What is left is the original chestnut coloration of the horse. Maverick is considered a mahogany bay because of his very dark red coat color. He also has a star and snip.


This is Ambrosia, a bay Thoroughbred/Trakehner cross.

The bay color is caused by the agouti gene. The agouti allele is expressed in writing as "A." Lowercase "a" means that the allele does not exist. It is possible for a horse to be homozygous for the agouti gene, or AA. In order for the horse to visibly be bay, however, the agouti allele must accompany a black allele. The horse can be homozygous for black and agouti (AA, EE), homozygous for one and heterozygous for another (Aa, EE or AA, Ee), or heterozygous for both (Aa, Ee). In all of these cases, the horse's phenotype will be bay.

If the agouti gene exists without a black gene, the horse will not have the appearance of the black having been restricted to the points. A chestnut horse can carry the agouti gene if one of its parents carried the agouti gene and it happened to be passed on. They can even be homozygous agouti if both parents carried the the gene and happened to pass it on. In writing, either case would be Aa, ee or AA, ee.

The agouti gene is also responsible for creating colors such as buckskin and perlino. More information on those colors is coming soon.


This is Ranger, a bay Tennessee Walking Horse. Note that his eye color is not brown but hazel. There is no genetic explanation for this as of yet, but some genes, such as the agouti gene, seem to carry a genetic marker for hazel eyes as well.


Above are Thunder and Bree. Thunder is a yearling Thoroughbred, and Bree is a Tennessee Walking Horse. Ambrosia is in the background. Bree also carries the tobiano gene, which is what creates her white markings. She is considered a bay tobiano. Obviously Thunder is still in his winter coat. As his owner has only recently acquired him as of this post, she isn't sure if his summer coat is going to be bay or brown (another modifier gene). We'll include more photos as we receive them.


This is Bree. We can clearly see that her legs, although white, still show the black coloration above the white.

Bay comes in many shades, most particularly mahogany bay, blood bay (a dark red color that is lighter than mahogany), light bay (a chestnut shade to the coat), and black or dark bay (a very dark red, very close to brown). There is no genetic difference in the shades; i.e., there is no way to know what shade the horse will be when breeding for bay.

Names for Bay Horses

The following are names I've gathered over the years that seem to describe the bay color of a horse very well. Feel free to respond so we can add to this list!

Ambrosia, Bayberry, Baylee, Bloodstone, Brandy, Brego, Burgundy, Catalina Bay, Cherry, Chianti, Cisco, Cleveland, Frisco, Garnet, Ginger, Jordan, Mahogany, Merlot, Rimrock, Robin, Seabiscuit, Scarlet, San Francisco ("Frisco"), Whiskey

Review:

Bay - caused by the agouti allele, which restricts the black coloration to the points of the horse.
Dominant - the qualities dominate the overall appearance of the horse but only if the black allele is also present.
Represented as AA (homozygous) or Aa (heterozygous).
Shades include mahogany bay, blood bay, light bay, and dark or black bay.

Thank you to Firewalkers Ranch for the photos of Ambrosia, Ranger, Thunder and Bree.