The Agouti gene controls the distribution of black pigment. This pigment can be either uniformly distributed or distributed to "points" of the body (ear rims, lower legs, mane, tail). Agouti has been linked to a deletion of 11 nucleotides in the agouti gene. The 11 nucleotide deletion of this gene is the recessive form of the gene. Only when the agouti gene is homozygous for the deletion (aa) is the black pigment evenly distributed. Heterozygous (Aa) or homozygous for the absence of the 11 nucleotide deletion (AA) results in point distribution of black pigment. Agouti has no effect on homozygous positive red factor (ee) horses as there has to be black pigment present for agouti to have an effect.
Agouti is not shown physically on red (ee) horses. Therefore, a breeder might want to test a chestnut base horse to see if it is an Agouti carrier. Testing bay horses might be desired to see whether the horse carries one (Aa) or two (AA) copies of the Agouti allele. A homozygous Agouti (AA) horse will always pass Agouti to its offspring whereas a heterozygous (Aa) horse will have a 50% chance of passing on the gene. Another reason to test for Agouti might be if there is some doubt whether a black horse is truly black or a very dark bay. The effects of other genes might also make it hard to tell if Agouti is present or not.
This test does not determine if a horse is homozygous for black factor. To determine black homozygosity, a breeder should test for red/black Factor. See the Red/Black Factor page for more info.
Animal Genetics offers DNA testing for the Agouti gene that controls the distribution of black pigment. The genetic test verifies the presence of the dominant Agouti mutation and presents results as one of the following:
|Bay||Only dominant allele detected. Black pigment distributed in point pattern. The horse cannot have black foals regardless of the color of the mate. The basic color of the horse will be bay or brown in the absence of other color modifying genes|
|Bay||Horse tested Heterozygous for Agouti. Black pigment distributed in point pattern. The horse can transmit either A or a to its offspring. The basic color of the horse will be bay or brown unless modified by other color modifying genes.|
|Black||Only recessive allele was detected. Black pigment distributed uniformly. The basic color of the horse will be black in the absence of other color modifying genes.|