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B-Locus (Black, 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 colour. A mutation in the TYRP1 gene can occur causing a change in function which dilutes the black colour pigment to a brown colour. 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, the dog's coat colour will be brown. This colour can also be referred to as liver, chocolate, or in some breeds, red.

B: controls the black coat colour

bs, bd and bc: control the brown coat colour.


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 colour 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 colour. 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 colouration, changing it from black to brown. Yellow lab puppies can have black or brown noses but Vizslas always have brown or flesh coloured noses.


Genotype Coat Colour Nose Colour Transmission of Coat Colour
EEbb brown (red)
brown-and-white (bicolour)
brown tricolour
Eebb brown (red)
brown-and-white (bicolour)
brown tricolour
brown red
eebb pure red
red-and-white (bicolour)
red tricolour
Genotype Coat Colour Nose Colour Transmission of Coat Colour
eeBB red
eeBb red
Black brown
eebb red

B Locus Testing:

Animal Genetics currently offers a test for the B-Locus to determine how many copies of the recessive (bs, bd, bc) allele a dog carries. Dogs can be DNA tested at ANY age.

Sample Type:

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-Allele Results:

B/B Black The dog carries two copies of the dominant B allele. The dog will have a black-based coat and will always pass on the "B" allele to any potential offspring. All offspring will also be black-based dogs.
B/b Black Both the dominant and recessive copies of the B allele are present. The dog will have a black-based coat 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 has a brown-based coat as well as a brown nose and foot pads. The dog will always pass on the recessive allele to all potential offspring.