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 for there has to be black pigment present for agouti to have an effect. Why test for Agouti?
There are several reasons an individual might want to test their horse for Agouti. 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 Factor. See the Red Factor
page for more info.