How does the B-locus create ‘red merle’ Great Danes?

The B-locus (specifically the TYRP1 gene) creates a “red merle” Great Dane by altering the color of the dog’s dark pigment (eumelanin) from black to a red or brown color.

Because the brown/chocolate allele (b) is a simple recessive trait, a dog must inherit two copies of it (bb)—one from each parent—for the color to be visually expressed. When a Great Dane possesses this homozygous recessive bb genotype, its underlying base coat becomes chocolate.

Crucially, this chocolate base color interacts with all other coat patterns the dog inherits. If a dog with the bb genotype also inherits the dominant merle gene, the merle mutation will act upon the red/brown pigment rather than the standard black pigment. This genetic combination produces a merle pattern composed of red/brown patches over a diluted background, creating what is known as a “red merle” (or chocolate merle) Great Dane.