Chocolate coat 2 (bs variant)

The chocolate 2 or bs variant is responsible for providing a chocolate brown color to the coat, eye rims, nose, paws and paw pads.

Definition

The so-called chocolate variant manifests itself in dogs as a deep brown or chocolate colouration of the coat. Dogs with this variant will also express brown pigmentation of the nose, footpads and eye rims. This phenotype results from alterations in the TYRP1 gene (also known as the B locus), which is involved in the production of the pigment responsible for black or brown colouration in dogs (eumelanin).

Genetic basis

The chocolate 2 variant is also known as the "bs" allele. It is necessary for the dog to inherit two copies of the c.991C>T variant to produce brown pigment instead of black. It is possible that there are other variants in the TYRP1 gene involved in the development of the trait which, in combination with the variant described here, result in the chocolate coloured coat. The presence of a single copy of the c.991C>T variant would not be sufficient for the expression of the phenotype mentioned, however, it can be passed on to the offspring of the dog with a 50% probability.

Other relevant information

There are other alternative names for this trait such as liver, brown or chestnut. The intensity of the chocolate color can vary from a deep dark chocolate color to a lighter brown. Breeds commonly showing the trait are Australian Shepherd, German Shepherd Dog, Lancashire heeler, Leonberger, Miniature American Shepherd and Siberian Husky.

Bibliography

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.