B-Locus (Brown, Liver, Chocolate)
TYRP1, or tyrosinase-related protein 1, is a protein that plays a role in the synthesis of the pigment eumelanin. In the dominant form of this gene, the "B" allele, normal eumelanin is produced in the coat and the dog's coat appears black in color rather than chocolate. A mutation in the TYRP1 gene can occur causing a change in function which dilutes the black color pigment to a brown color. This mutated gene is known as the "b" allele. When a dog is homozygous for the mutation, meaning he has two copies of the recessive allele, all black pigment appears brown. This color can also be referred to as liver, chocolate, or in some breeds (such as the Australian Shepherd), red.
The black (B) allele is dominant to the brown alleles (bs, bd, bc). In this gene are three common mutations (bs, bd, bc) which result in brown instead of black eumelanin production. However, some breeds have additional mutations that cause chocolate coat color that have not been identified. For example, the mutation for chocolate in French Bulldogs has not been determined.
Because TYRP1 is only associated with eumelanin, this mutation only has an effect on coat color of dogs that are "EE" or "Ee" at the E-Locus. Dogs that are "ee" only produce phaeomelanin in their coats, so a mutation at the B-locus will not have an effect on their coat color. However, eumelanin is still produced in the foot pads and noses of dogs which are yellow to red (e/e at MC1R) so the B-Locus still has an effect on these areas. Dogs that are "eebb" will have a brown nose and foot pads, rather than black. TYRP1 mutations affect the nose and pad coloration, changing it from black to brown. Yellow lab puppies can have black or brown noses but Vizslas always have brown or flesh colored noses.
B Locus Testing:
Animal Genetics currently offers a test for the B-Locus to determine how many copies of the recessive "b" allele a dog carries. Dogs can be DNA tested at ANY age.
Animal Genetics accepts buccal swab, blood, and dewclaw samples for testing. Sample collection kits are available and can be ordered at Canine Test Now.
Testing Is Relevant For The Following Breeds:
Most breeds except French Bulldogs and some Beagles.
Animal Genetics offers DNA testing for dominant B allele. The genetic test verifies the presence of the mutation and presents results as one of the following:
|B/B||Black||The dog carries two copies of the dominant B allele. The dog will express black pigment rather than chocolate, and will always pass on the "B" allele to any potential offspring.|
|B/b||Black||Both the dominant and recessive copies of the B allele are present. The dog will express black pigment rather than brown, but carries the allele responsible for the brown phenotype. The dog can pass on either allele to potential offspring.|
|b/ b||Brown||Two copies of the recessive allele are present. The dog will express brown pigment rather than black as well as a brown nose and foot pads. The dog will always pass on the recessive allele to all potential offspring.|