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The chocolate coat trait is characterized by the chocolate brown pigmentation of the coat, in addition to the nose, paw pads, nails, and eye rims. The variant has been identified in the TYRP1 gene.

Definition

The variant known as chocolate manifests in dogs as a deep brown or chocolate coloration of their coat. Dogs that have this genetic variant in the TYRP1 gene will also express brown pigmentation of the nose, paw pads, nails, and eye rims. In addition, their eye color will generally be lighter (yellow, honey, amber, etc.) compared to black dogs. This phenotype is a consequence of alterations in the TYRP1 gene (also known as the B locus), which plays a key role in the biosynthesis of eumelanin (black pigment) and in the maintenance of melanosomes. Chocolate fur results from a loss-of-function mutation in the TYRP1 gene, which prevents the normal production of eumelanin, leading to the formation of brown or chocolate pigment instead of black pigment.

Genetic basis

At least six different genetic variants affecting the normal function of the TYRP1 gene have been identified. In this study, we analyzed three of the best-known variants: bc allele (c.121T>A), bs allele (c.991C>T), and bd allele (c.1033_1035del). The chocolate coat trait follows an autosomal recessive inheritance pattern, meaning that two copies of one of these three variants are needed for the dog to produce chocolate brown pigment instead of black. Furthermore, chocolate coloring may be expressed if the dog has one copy of two different variants, a phenomenon known as compound heterozygosity. The presence of a single copy of the bc, bs, or bd alleles will not cause the chocolate coat, but it can be passed on to offspring with a 50% probability. However, the chocolate phenotype may be expressed if any of these variants is combined with another variant not analyzed in this study.

Other relevant information

This trait is also known by other names, such as liver, brown, or chestnut. The intensity of the chocolate color can vary, from a dark and intense chocolate tone to a lighter brown. Breeds such as the Australian Shepherd, German Shepherd, Lancashire Heeler, Leonberger, Miniature American Shepherd, and Siberian Husky often exhibit this trait more frequently. Interestingly, in the Vizsla breed, most dogs have a brown nose, regardless of the presence or absence of variants in the TYRP1 gene.

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.