What causes the ‘red’ color in a red merle?

The “red” color in a “red merle” is caused by a recessive trait at the B-locus (specifically the TYRP1 gene) that alters the dog’s dark pigment (eumelanin) from black to a red or brown color.

Because the brown (or chocolate) allele (b) is a simple recessive trait, a dog must inherit two copies of it to possess a homozygous bb genotype in order for the red/brown color to be visually expressed. When a dog with this bb genotype also inherits the merle gene, the merle mutation acts upon the red/brown pigment instead of the standard black pigment. This genetic interaction creates a mosaic pattern of fully pigmented red/brown patches over a diluted background, producing what is known as a “red merle.”