A-Locus (Fawn/Sable; Tricolor/Tan Points; Solid Black)

Description

The Agouti gene (A-Locus) controls several common coat patterns in dogs, including fawn and sable, wild sable, black-and-tan (tan points), tricolor and recessive black. Four alleles of the gene have been identified: a^y, a^w, a^t and a. If you’d like to learn more about the difference between an allele and a gene, click here.

These alleles follow a dominance hierarchy: a^y is dominant over a^w, a^w is dominant over a^t, and a^t is dominant over a. For example, a dog that is a^y/a^t will show the color associated with a^y, not the color associated with a^t.

The A-Locus is only expressed in dogs that are k^y/k^y at the K-Locus. A dog that carries dominant black at the K-Locus, or two copies of the recessive allele at the E-Locus, will not show its agouti pattern, because both of those genes override the A-Locus. The dog still carries its A-Locus alleles and can pass them on to its offspring.

The color produced by the A-Locus can also be modified by other genes, such as the B-Locus or D-Locus. For example, a dog that is b/b (recessive) at the B-Locus will have chocolate pigment in the areas that would otherwise be black. A fawn dog that is b/b will have a chocolate nose, and a dog that is a^t/a^t and b/b will be chocolate and tan rather than black and tan.

In the sections below, a dog described as n/n for an allele has no copies of that allele.

The “a^y” Allele

The a^y allele is the most dominant of the four alleles on the A-Locus. It produces a range of coat colors, from light fawn to darker red to sable, depending on how the gene is expressed. Dogs that are k^y/k^y at the K-Locus and have one or two copies of the a^y allele will always show the a^y coat pattern. A fawn or sable dog can also carry any one of the other three alleles. That allele is not expressed, and it is not possible to tell from the dog’s appearance that it is there.

A fawn dog will not always pass on the a^y allele, or its own coat color, to its puppies. A dog that is a^y/a^w, a^y/a^t or a^y/a has a 50% chance of passing on the a^y allele and a 50% chance of passing on the other allele. A dog with two copies of the a^y allele will always pass on a^y. As long as that dog is bred to another dog that is k^y/k^y at the K-Locus, it will always produce fawn or sable puppies.

The “a^w” Allele

The a^w allele produces a color known as “wild sable.” This coloration is sometimes called the “wild type” or, in some breeds, “wild boar.” The individual hairs are banded, switching from black pigment to a reddish or fawn color. It is sometimes seen in German Shepherds and other shepherd breeds. The a^w allele is recessive only to a^y, so it is dominant over the a^t and a alleles.

A dog that is n/n for the a^y allele and has one or two copies of the a^w allele will show the a^w coloration. A dog with one copy of a^w can also carry either the a^t or the a allele without showing it. Even though that allele does not appear in the coat, it can be passed on to any offspring.

The “a^t” Allele

Both the “black-and-tan” and “tricolor” phenotypes (expressed traits) are caused by the a^t allele. A tricolor dog is black-and-tan with white. White is generally an absence of color rather than a pigment the dog produces. For a dog to be black-and-tan or tricolor, it must be k^y/k^y at the K-Locus, so that dominant black is not expressed, and it must have either two copies of the a^t allele or one copy of a^t and one copy of a.

The dog must also be n/n for both the a^y and a^w alleles, because both are dominant over a^t. A dog that is a^t/a^t will always pass on a copy of the a^t allele. This does not guarantee black-and-tan puppies, because the coat color of the offspring also depends on the genotype of the other parent.

The “a” Allele

The a allele is sometimes called the recessive black gene. It is the most recessive of the four alleles, so a dog must have two copies (a/a) and be n/n for a^y, a^w and a^t to show it. A dog that is k^y/k^y at the K-Locus and a/a at the A-Locus will be solid black. This is generally the cause of a solid black German Shepherd.

It is important to distinguish recessive black from dominant black. A dog that is K^B/K^B or K^B/k^y at the K-Locus is also solid black, but that color does not depend on the A-Locus. You can learn more about this type of dog by reading about the K-Locus and the B-Locus. A recessive black dog will always pass on the a allele to all of its offspring.

Possible results

Reference

Identification of a mutation that is associated with the saddle tan and black-and-tan phenotypes in Basset Hounds and Pembroke Welsh Corgis.

[PMID: 23519866]

Characterization of the dog Agouti gene and a nonagouti mutation in German Shepherd Dogs.

[PMID: 15520882]