This webpage is part of a series on Dog Coat Color Genetics and was last updated on February 3, 2013 by Sheila Schmutz
Recent studies show that the agouti signal peptide (ASIP) competes with melanocyte stimulating hormone (MSH), which produces eumelanin pigments, to bind on the melanocortin receptor and must sometimes win. Both the E allele and Em allele are responsive to agouti or melanocortin binding in dogs. However dogs that are e/e have a mutation in MC1R and produce only phaeomelanin. In such dogs, the agouti genotype doesn't affect their coat color, which will be some shade of cream, yellow or red.
To further complicate things, agouti has more than one promoter which seems to signal where on the body, or even on individual hairs, each pigment is laid down. Roughly, one seems to control ventral or belly color and another dorsal or back color. The simplest way to "see" this is on a black and tan dog......the back is black from eumelanin pigment being made and the belly is tan or red from phaeomelanin pigment being made. In some dogs banded hairs are produced over parts of the body. With certain genotypes, the coat color changes from birth to adulthood, usually being born darker and then lightening.
In about 2005 we mapped the agouti gene to dog chromosome 24. This gene undoubtedly has several alleles, but how many is still an open question. Some have been identified using DNA studies and tests for agouti phenotypes in some breeds. Although several books attempt to state the dominance hierarchy of the agouti alleles, until recently no breed was proven to have all the alleles. Most books suggest that it is ay > aw > at > a. Breeding data and DNA data from our collaborative study with Dr. Greg Barsh's group at Stanford supports this. The Eurasier is one breed that has all four alleles (Dreger and Schmutz 2011).
Wild Type Black Banded Hairs (aw)
Agouti signal peptide is also the gene that causes a wolf or coyote to have yellowish hair with black tips and base, or what is often called banded. The competition, in that case, is going on as the hair is growing, which results in a hair that changes color along its length. In breeds with very long hair there may be even more alternating bands of phaeomelanin and eumelanin. This gene is likely also causing the change in hair color in the Malamute, Siberian Husky (shown above), the Elkhound, and some German Shepherd Dogs. The allele which causes this banding of hairs is sometimes called the wild type allele (aw), or wolf sable. In German Shepherds, like Fello at the right, this pattern is called sable. Although this allele was once fixed in dogs, if we assume that they all descended from the wolf, it is not a very common allele in dog breeds today.
Fawn or Clear Sable (ay)
Another allele, ay, has been postulated by Little to explain fawn Whippets, Great Danes, and Pugs which are yellow in pigmentation over all of their body or if masked, most of the body. Latte, the Great Dane, illustrates fawn without mask and Pansy, the Pug, shows it with mask. In some breeds such as Basenjis, Dachshunds, and French Bulldogs there may be dogs that are "clear red" with the genotype "e/e" and others that are "fawn" ayay. Some breeds have black tips on their hairs into adulthood, like Pugs. Other breeds have large black tips on the hairs in puppyhood that grow out by the time the dog matures.
Fawn (ay) is considered dominant to black-and-tan (at) and the wild type allele (aw).
This same ay allele is responsible for the coat color called sable in Tervuren, Malinois, Lakenois, Collie, and Shetland Sheepdog. In all these breeds some solid black hairs occur and a few hairs with alternating bands of phaeomelanin and eumelanin can be present. The sable, and probably all red coat colors of the Cardigan Welsh Corgi are also caused by this allele. Note that sable in German Shepherd Dog is a term used to describe the presence of predominantly banded hairs and is not caused by this allele, but instead by the aw allele as described above. Distinguishing the difference between Lady, the Shetland Sheepdog and Fello, the German Shepherd Dog above, is not simple from a photo or even from a distance in person.
Although many dog breeders have assumed that the differences in darkness of fawn or sable dogs is determined by whether they are homozygous or heterozygous for ay, it is now known that some to all of this variation is caused by another gene. The cover from the 2005 issue containing our article describing the ay allele shows 4 dogs that are all ay/ay, yet vary tremendously in shading: Peyton, a female Shetland Sheepdog; Rain, a male Belgian Tervuren; Bull, a male Mastiff; Ginny, a female Akita. DNA tests to distinguish this allele from the "a" in Shelties, Tervuren/Groenendael, and a few other breeds are now offered by Healthgene (http://www.HealthGene.com), based on these findings.
Brindle dogs which are full-body brindles, as opposed to dogs with only brindle undersides, have at least one ay allele.
Tricolor, Black-and-Tan, Tan Points (at)
|This Gordon Setter is an example of a breed that is black-and-tan. C. C. Little (1957) would say these dogs were atat. Tan Points are more difficult to distinguish on this liver French Brittany but still visible. On an orange English Setter, they can be impossible to detect.|
|This Staffordshire Bull Terrier is black-and-brindle. Priscilla has brindle where other dogs would have tan because she has at least one copy of the brindle allele and a genotype of atat at the agouti locus.|
The tricolor collie pup and Dachsbracke above are examples of dogs that are black-and-tan with white. Willis seems to suggest another similar allele as for saddle. He would use this allele to describe the Dachsbracke or a Beagle, rather than black-and-tan. Recent studies in our lab, presented at the conference on Advances in Canine and Feline Genomics and Inherited Disease in Baltimore, MD from Sept. 23-25, 2010, indicate that all saddle tan dogs have an at/at or at/a genotype. Saddle tan appears to be a modification of the black-and-tan phenotype caused by interaction of other genes with ASIP.
Most often the tan is located in specific body regions or points and called "tan point" as in the pup above left. Occasionally an interaction with another gene disrupts the location of the tan and gives tan patches that are more randomly located, as in its merle littermate on the right.
Recessive Black (a)
Some German Shepherd Dogs, like Flash on the left, are solid black due to a recessive genotype that is relativley rare in dogs in general but is common in this breed. Little did not suggest such a "recessive black" allele, but Carver (1984) did document it in his study of German Shepherd dogs. This allele which causes a recessive black coat color (a) is also the black of Shetland Sheepdogs. It also occurs in the Schipperke, American Eskimo Dog, Samoyed and Puli, as shown by Kasa on the right.
|Both this black, and the more common black, occur in Australian Shepherds and in the Groenendael variety of Belgian Sheepdogs as shown by Ralph on the right.|
Originally shown on: Dreger, D. L. Kaelin, C. B., Barsh, G.S., Schmutz, S.M. Proposed evolution of the agouti locus alleles in domestic dogs. 5th International Canine and Feline Genome and Hereditary Disease Conference, Baltimore, MD Sep 22-25, 2010
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