How does the B-locus interact with other patterns?

The B-locus (specifically the TYRP1 gene) determines whether a dog will have a standard black base or a brown (chocolate) base. Because the recessive chocolate allele (b) alters the dog’s dark pigment (eumelanin) from black to a red or brown color, it interacts with and impacts all other coat colors and patterns a dog might inherit.

Here is how the B-locus interacts with various Great Dane patterns:

• Fawns and Brindles: The B-locus affects any specific areas of the coat that are genetically coded to be black. In a Fawn Dane, this means the standard black mask will turn brown. In a Brindle Dane, both the mask and the characteristic striping will be altered to brown, creating a “chocolate brindle”.

• Merles and Harlequins: When a dog possesses the homozygous recessive bb genotype alongside the merle or harlequin genes, those pattern modifiers will act upon the brown pigment instead of black. This interaction produces chocolate harlequins and “red merle” (chocolate merle) Great Danes.

• Dilute (D-Locus) Interaction: The B-locus also interacts directly with the D-locus. If a dog with a chocolate base coat also inherits two copies of the recessive dilute gene (d/d), its brown pigment will be further lightened, creating a lilac or silver colored coat.