Gene TYRP1 (Tyrosinase related protein 1) is responsible for chocolate/liver brown colour in dogs. This coat colour is inherited recessively. There are 4 different alleles on B locus: normal B allele (dominant), and three recessive alleles bc, bs and bd. Combination of any two recessive alleles will result in chocolate/liver brown colour (for example two bc alleles or one bc and one bs allele).
Because TYRP1 gene is related to eumelanin production, it only affects dogs which have genotype E/E or E/e on E locus. B locus also affects colour of the nose, eyes and pads. Dogs with genotype e/e produce only pheomelanin in coat but production of eumelanin in the nose, eyes and pads is not affected and therefore totally controlled by B locus. Depending on genotypes of E and B loci dog can have yellow coat and brown nose/eyes (e/e, b/b) or chocolate coat and brown nose/eyes (E/e or E/E and b/b).
Test for B locus is divided into three separate tests in order to find the presence of recessive alleles. Results are presented as combined results as well as results for each single allele tested.
|B/B:||dog is homozygous for normal allele; no recessive alleles for chocolate/liver brown are present, dog will transfer dominant alleles to the entire of its offspring.|
|B/b:||dog is heterozygous for chocolate/liver brown when one of recessive alleles is present (for example Bc/bc, Bs/Bs, Bd/Bd), brown colour is not expressed, one recessive allele can be transferred to offspring.|
|b/b:||dog is chocolate/liver brown (homozygous for brown colour) in eumelanin pigmented areas and carries at least two recessive alleles (identical or different), which means that dog is either homozygous for one allele (for example bc/bc, Bs/Bs, Bd/Bd) or heterozygous for two alleles (for example Bc/bc, Bs/bs, Bd/Bd).|