The Agouti gene controls the distribution of black pigment and determines whether a horse will have a bay or black coat. The effect of the agouti gene is to be combined with the extension gene to obtain the three basic coat colours. Horses carrying the E allele of the extension gene are black if they have two copies of the a allele of the agouti gene and bay if they have at least one copy of the A allele of the agouti gene.
