October 23, 2008
The most common coat color modifier makes bay horses.
By Andrea Caudill
All horse colors are either black- or red-based, and all other colors are derivatives of these. The most common modifier is the agouti (ah-GOO-tee) gene. The Spanish-derived word comes from a South American rodent related to a guinea pig.
The agouti modifier is a portion of a gene called an allele. It produces bay horses by restricting the black color to only the horse’s points. A chestnut horse can also carry the agouti gene, but because he doesn’t have black pigment, the horse’s phenotype will not be affected – in other words, you can’t tell by looking at it.
The shades of bay can range from very pale (not to be confused with buckskin, which is a bay horse with a cream dilution) to dark bay (which can be confused with brown). A bay horse can sometimes appear to have a dorsal stripe, which is known as counter shading.
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The agouti gene is dominant, which means that if the horse has it and carries the black gene, it will be expressed. Thus, a bay horse must have at least one parent that is a black-based color.
A brown horse, which is a near-black horse with brown points (muzzle, underside of belly) is more a phenotype than a specific genetic color, as it can be caused by several genetic controls. It might be genetically black with shading or sun-bleaching to cause the brown, or the color might be caused by the agouti gene (genetically bay) with smuttiness added, causing the horse to darken. They look nearly black, showing just hints of brown around the muzzle, flanks and the insides of the legs.
A modification of a bay horse called “wild bay” restricts the black points mostly to the pastern and below, although occasionally there’s some black on the cannon bone. In the common bay, the black points on the leg extend to the knee and hock or above. The modification is relatively rare.
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In 2007, 19 percent of American Quarter Horses registered were bay.
Agouti is dominant, so depending on if a horse is homozygous or heterozygous, it has a 50-100 percent chance of passing the agouti gene to its offspring.
Color legends identify bays as gentle, steady and durable horses.
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