Genetic basis of B Locus - corresponds to the TYRP1 gene (Tyrosinase-Related Protein 1), which controls the production of eumelanin pigment responsible for black or brown coat, nose, and footpad color.The dominant allele B produces black pigment, while recessive alleles b (including bd, bs, bc variants) result in brown pigment when a dog inherits two copies (homozygous recessive). The three common brown alleles tested are: bd: a 3 base pair deletion, bs: a point mutation (C>T), bc: a T>A substitution. These alleles can be present in various breeds including Labrador Retrievers, Doberman Pinschers, Poodles, Australian Shepherds, Border Collies, and many others. Dogs with two copies of any combination of these recessive b alleles (e.g., bd/bd, bs/bs, bc/bc, or bd/bs, etc.) will have brown coat color, nose, and footpads. Dogs with only one copy and one dominant B allele (e.g., B/bs or B/bd) will usually appear black but are carriers of brown.
Inheritance Pattern - The brown color phenotype at the B locus is autosomal recessive. A dog must inherit two recessive alleles (any combination of bd, bs, or bc) for brown coloration to be expressed. One copy results in the dog being a carrier, which looks black but can pass the brown allele to offspring. Breeding two carriers results in a 25% chance of brown puppies.
Phenotypic effects - Dogs homozygous for brown alleles commonly have a brown or liver-colored coat, along with brown noses, paw pads, and eye rims.This change affects eumelanin-based pigments only; phaeomelanin (red/yellow pigment) is unaffected. Brown is sometimes referred to as chocolate or liver color, and some breeds have unique names for it (e.g., "red" in Dobermans, Australian Shepherds). Expression of brown coat color also requires proper genotype at the E locus (MC1R) and K locus (CBD103). For example: Dogs must be E/E or E/e at the E locus to express eumelanin color. Dogs must be ky/ky at the K locus for expression of the B locus alleles.
Genetic Testing - Genetic testing often covers these three main variants (bd, bs, bc) to assess B locus status. Test results may show combinations like (bd/bs), (bc/bs), (bd/bd), etc. Interpretation requires knowing which alleles are inherited together, which can sometimes be ambiguous without parental genotypes. Breeders use these tests to predict offspring coat color and avoid unexpected brown litters.