B locus (Brown)

The Paw Print Genetics B locus (Brown) coat color test evaluates the TYRP1 gene for all three brown variants (bc, bd, and bs) to determine the overall B locus genotype for the dog. Dogs with B/B and B/b genotypes can have a black coat, whereas dogs with a b/b genotype can have a brown coat. However, the dog's coat color is also dependent on the dog's genotypes at the E, K, and A genes among others. The B locus also determines the color of the dog's nose and foot pads, regardless of the dog's genotypes at the E, K, and A genes.
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Testing Summary

The B Locus (Brown) coat color test reliably determines if a dog has one of the following genotypes at the B locus:

B/B

This dog carries two copies of B at all three of the bc, bd, and bs loci making the overall B locus genotype of this dog B/B. The overall B locus genotype for a dog is determined by the combination of the genotypes at the bc, bd, and bs loci. The bc, bd, and bs variants confer brown coat, nose, and foot pads when at least one of these DNA changes is present on both genes of the dog at the B locus. If the dog has one or no copies of b then the dog will have a black coat, nose, and foot pads. However, this dog’s coat color is also dependent on the E, K, and A genes. This dog will pass on B to 100% of its offspring.

Interpretation: Black coat, nose and foot pads
B/b

This dog carries one copy of B and at least one copy of b at the bc, bd or bs locus making the overall B locus genotype of this dog B/b. The overall B locus genotype for a dog is determined by the combination of the genotypes at the bc, bd or bs loci. The bc, bd, and bs variants confer brown coat, nose, and foot pads when at least one of these DNA changes is present on both genes of the dog at the B locus. If the dog has one or no copies of b then the dog will have a black coat, nose, and foot pads. However, this dog’s coat color is also dependent on the E, K, and A genes. This dog will pass on B to 50% of its offspring and b to 50% of its offspring.

Interpretation: Black coat, nose and foot pads (carries brown)
b/b

This dog carries two copies of b at the bc, bd or bs locus making the overall B locus genotype of this dog b/b. The overall B locus genotype for a dog is determined by the combination of the genotypes at the bc, bd and bs loci. The bc, bd, and bs variants confer brown coat, nose, and foot pads when at least one of these DNA changes is present on both genes of the dog at the B locus. If the dog has one or no copies of b then the dog will have a black coat, nose, and foot pads. However, this dog’s coat color is also dependent on the E, K, and A genes. This dog will pass on b to 100% of its offspring.

Interpretation: Brown coat, nose and foot pads

Detailed Summary

Variants of the TYRP1 gene at the brown (B) Locus impact canine coat color by disrupting regular eumelanin (black pigment) synthesis which causes the dilution of black pigmented areas to brown. The B locus also controls the color of the dog’s nose and foot pads regardless of the dog’s genotype at the E locus because eumelanin is still produced in the epidermis of the nose and foot pads of e/e dogs. Although black is dominant to brown, the overall B locus genotype for a dog is determined by the combination of the genotypes at the bc, bd and bs loci. The bc, bd, and bs variants confer brown coat, nose, and foot pads when at least one of these DNA changes is present on both genes of the dog at the B locus. If the dog has one or no copies of b then the dog will have a black coat, nose, and foot pads. However, the dog’s coat color is also dependent on the E, K, and A genes among others.


Testing Tips

The Paw Print Genetics B Locus (Brown) coat color panel tests all three brown loci (bc, bd and bs). Located at the bottom of the known canine coat color genetic interaction flowchart, the brown (B) locus interacts closely with the E, K, and A genes to produce the dog’s coat color. Because of the combined interaction of the three brown loci, black dogs can be carriers of brown genes that produce brown puppies. Genetic testing of all three brown loci will accurately determine whether a dog is a genetic Carrier of black or brown coat color. Note: In situations where two or three single copy variants of bc, bd, and bs are identified in a dog, the presence of multiple variants on a single copy of the gene cannot be excluded. Therefore, the overall B locus genotype for a dog could be B/b or b/b and cannot be determined by the laboratory without additional testing of parental samples. However, in this kind of situation, evaluating the color of the dog's nose will indicate whether or not the dog has a B/b genotype (black nose) or a b/b genotype (brown nose).


References

  • Schmutz SM, Berryere TG, Goldfinch AD. TYRP1 and MC1R genotypes and their effects on coat color in dogs. Mamm Genome. 2002 Jul; 13(7):380-7. [PubMed: 12140685]