How do breeders use the B-locus for chocolate colors?

The B-locus (specifically the TYRP1 gene) determines whether a Great Dane will have a standard black base coat or a brown (chocolate) base coat.

Here is how breeders use the B-locus to understand and produce chocolate colors:

• Understanding the Recessive Trait: The chocolate gene (b) is recessive, meaning it alters the dog’s dark pigment (eumelanin) from black to a red or brown color. Because it is a simple recessive trait, a breeder must ensure a puppy inherits two copies of the chocolate gene (bb)—one from each parent—in order for the dog to visually express a chocolate coat.

• Applying it to Other Patterns: The chocolate base can interact with and impact all other coat colors and patterns. Breeders can use the B-locus to produce unique combinations such as chocolate harlequins, chocolate fawns, chocolate brindles, and “red merles,” or to create brown masks and stripes on fawn and brindle dogs. Furthermore, if a chocolate dog also inherits two copies of the recessive dilute gene at the D-locus, its chocolate coat will be further lightened to lilac or silver.

• Predicting Litter Outcomes: Breeders use DNA testing at the B-locus to uncover if their dominant black dogs are secretly carrying the recessive brown trait. Because simple recessive traits are highly predictable, breeders can reliably calculate litter outcomes once they know the parents’ genotypes. For example, if a breeder has a waitlist for chocolate and lilac puppies, they can mate a chocolate male (that carries the dilute gene) with a lilac female, which will statistically guarantee a litter of 50% chocolate puppies and 50% lilac puppies.