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Breeds: All
Description
The B-Locus test looks for the variants that cause brown coat color in dogs, also called liver or chocolate. Tyrosinase-Related Protein 1 (TYRP1) is a protein that plays a role in making the pigment eumelanin. This pigment darkens a dog’s coat, creating black or brown coats. In the dominant form of the TYRP1 gene (the B-Locus), enough eumelanin is produced that the dog’s coat appears black instead of chocolate. This dominant form is the “B” allele.
A mutation in the TYRP1 gene changes the production of eumelanin and dilutes the black pigment to brown. The mutated form is known as the “b” allele. When a dog is homozygous for the mutation, meaning it has two copies of the recessive allele (b/b), all black pigment appears brown. This color is also referred to as liver or chocolate, and in some breeds (such as the Australian Shepherd) it is referred to as red. Because the “B” allele is dominant over the “b” allele, a dog that is B/b or B/B will have a black coat, not chocolate. The dog must be b/b to show the brown coloration.
There are three common brown alleles: bs, bd and bc. The black (B) allele is dominant over all three. A dog with no copies of the “B” allele is b/b and can carry any combination of bs, bd and bc. All of these result in brown instead of black. It is important to note that some breeds have additional mutations that can cause a chocolate coat color. For example, there is a form of chocolate in French Bulldogs that is caused by a nonsense variant in HPS3 and is referred to as “cocoa” or “non-testable chocolate.”
Because TYRP1 only affects eumelanin, the B-Locus only changes the coat color of dogs that are E/E or E/e at the E-Locus. Dogs that are e/e produce only phaeomelanin in their coats, which gives yellow to red colors and does not produce the darker coat colors. In an e/e dog, a mutation at the B-Locus will not have an effect on coat color.
However, eumelanin is still produced in the nose and foot pads of e/e dogs, so the B-Locus still has an effect on these areas. A dog that is e/e and b/b will have a brown nose and brown foot pads rather than black. This is why yellow Labrador puppies can have black or brown noses, while the Vizsla always has a brown or flesh-colored nose.
Possible results
| Genotype | Description |
|---|---|
| B/B | No brown: Dog does not carry the mutation for most forms of chocolate coloration. |
| B/b | Carrier: Dog carries one copy of the gene responsible for chocolate/brown coloration. |
| b/b | Brown/chocolate: Dog has two copies of the brown/chocolate gene. |
Reference
TYRP1 and MC1R genotypes and their effects on coat color in dogs.
[PMID: 12140685]