Each horse has a basic colour, which can be black, bay/brown or chestnut. These basic coat colours are controlled by the Extension and Agouti genes. The Agouti gene (A-locus) controls the distribution of black pigment. The pigment can be uniformly distributed or distributed to the “points” of the body (mane, tail, lower legs and inside the ears). The Agouti gene has no effect on horses that are homozygous ee for the Extension gene as black pigment has to be present for agouti to have an effect. The Coat Colour Agouti test (P907) tests for the genetic status of the Agouti gene. The Agouti gene has two variants (alleles). The dominant allele A restricts black pigment to the points of the horse (for example in bays and buckskins) and the recessive allele a uniformly distributes black pigment over the entire body. Only when the horse has two copies of the recessive allele a (homozygous aa), the black pigment is evenly distributed. The black pigment is distributed into the points if at least one copy of the allele A is present. All horses, regardless of their coat colour do have the genetics for the distribution of black pigment, but it’s not always physically visible.
The Coat Colour Agouti test encloses the following results, in this scheme the results of the Coat Colour Agouti test are shown in combination with the possible results for the Coat Colour Chestnut test:
|
Result Agouti |
Result Chestnut |
Coat Colour |
Description |
|---|---|---|---|
|
a/a |
E/E or E/e |
Black |
Only the recessive allele a was detected. The black pigment is distributed uniformly. If the horse is not e/e for the Extension gene, the basic colour is black unless modified by other colour modifying genes. It can only pass on allele a to its offspring. |
|
a/a |
e/e |
Chestnut, Sorrel |
Only the recessive allele a was detected. The black pigment is distributed uniformly. Because the horse is e/e for the Extension gene, the basic colour is chestnut or sorrel unless modified by other colour modifying genes. It can only pass on allele a to its offspring. |
|
A/a |
E/E or E/e |
Bay, Brown |
The horse is tested heterozygous for Agouti. The black pigment is distributed into the points. If the horse is not e/e for the Extension gene, the basic colour is bay or brown unless modified by other colour modifying genes. It can pass on either allele A or a to its offspring. |
|
A/a |
e/e |
Chestnut, Sorrel |
The horse is tested heterozygous for Agouti. The black pigment is distributed into the points. Because the horse is e/e for the Extension gene, the basic colour is chestnut or sorrel unless modified by other colour modifying genes. It can pass on either allele A or a to its offspring. |
|
A/A |
E/E or E/e |
Bay, Brown |
Only the dominant allele A was detected. The black pigment is distributed into the points. If the horse is not e/e for the Extension gene, the basic colour is bay or brown unless modified by other colour modifying genes. It can only pass on allele A to its offspring and therefore cannot produce black foals. |
|
A/A |
e/e |
Chestnut, Sorrel |
Only the dominant allele A was detected. The black pigment is distributed into the points. Because the horse is e/e for the Extension gene, the basic colour is chestnut or sorrel unless modified by other colour modifying genes. It can only pass on allele A to its offspring and therefore cannot produce black foals. |
