B-Locus (Brown/Chocolate/Liver)

Description

Tyrosinase-Related Protein 1 or TYRP1 is a protein that plays a role in the creation of the pigment eumelanin. This pigment is what causes the darkening of a dog’s coat color, creating black or brown coats. In the dominant form of the TYRP1 gene (aka the B-Locus), enough eumelanin is produced so that the dog's coat appears black instead of chocolate. This dominant form appears when the dog has the “B” allele.

A mutation in the TYRP1 gene can occur, causing a change in the production of the eumelanin. This dilutes the black color pigment into 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 (b/b), all black pigment appears brown. This color can also be referred to as liver or chocolate. In some breeds (such as the Australian Shepherd), this color is referred to as red. However, the "B" allele is dominant over the "b" allele. Therefore, a dog that is B/b or B/B will have a black coat, and not chocolate. The dog must have b/b in order to have a variation of the brown coloration.

The black (B) allele is dominant to the brown alleles (b^s, b^d, b^c). If the dog is negative (n/n) for the dominant "B" allele, the dog will then be b/b and can display the colorations of b^s, b^d, or b^c. These are three common mutations of the b/b allele. These will result in brown colors, instead of the black coloration the eumelanin produces. It’s important to note that some breeds have additional mutations that can cause the 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 is only associated with eumelanin, this mutation only has an effect on coat color of dogs that are "E/E" or "E/e" at the E-Locus. In order for a dog to have the opportunity to display colorations related to the B Locus, the dog must first have the dominant form on the E Locus (E/E or E/e). Dogs that have the e/e allele only produce phaeomelanin in their coats. Phaeomelanin does not produce the darker coat colors. If a dog does not have E/e or E/E, 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.  These can appear yellow to red if recessive (e/e at MC1R), so the B-Locus still has an effect on these areas. Dogs that are e/e: b/b will have a brown nose and brown foot pads, rather than black. TYRP1 mutations can affect the nose and pad coloration, changing it from black to brown. Yellow lab puppies can have black or brown noses. However, the Vizsla breed will always have brown or flesh-colored noses.