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.
Animal Genetics offers
DNA testing and detection of the Agouti bay/black pigment distribution.
Collect sample by pulling (not cutting) 20-30
mane or tail hairs with roots attached. It is important that you
pull the hairs and confirm that the actual root of the hair is being
collected. The root contains the genetic material of your horse
that is needed for DNA testing. Therefore, cut hairs do not provide
an adequate sample of your horse's DNA. Place the collected hairs
of each horse in separate zip-lock bags labeling the bags accordingly
with the horses name or identification number. Download and complete
a submission form for each sample and send along with payment to
Animal Genetics for testing.
Results are given using the following symbolic
|Only recessive allele
detected. Black pigment distributed uniformly. The basic color
of the horse will be black in the absence of other color modifying
|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.
|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.