The Tyrosinase-Related Protein 1 (TYRP1) gene, also known as Brown gene or B-Locus controls the dilution from black pigment to brown. The TYRP1 gene has no effect on the hair colour of dogs that are homozygous ee for the E-Locus as they do not have black pigment, but does have an effect on the colour of the nose and foot pads of these dogs. The Coat Colour B-Locus (H733) tests for the genetic status of the B-Locus. The B-Locus has four variants (alleles). The B allele is dominant and does not dilute the black pigment. From the recessive b allele three variants exist bs, bd and bc. All three variants of the recessive b allele have the same effect resulting in dilution of the black pigment into brown/chocolate/liver. Only when the dog has two copies of the recessive allele b (homozygous bb) the black pigment will be diluted to brown/chocolate/liver. For dogs that are red/yellow/cream and carry two copies of the recessive allele b the hair colour is not diluted but the colour of the nose and foot pads is changed from black to brown. In some breeds other mutations are present that cause chocolate colour that have not been identified yet.
The Coat Colour B-Locus test encloses the following results, in this scheme the results of the Coat Colour B-Locus test are shown in combination with the possible results for the E-Locus):
|B-Locus||E-Locus||Coat Colour||Nose/foot pads|
|B/B||Em/Em, Em/E or Em/e||Black, melanistic mask is not visible||Black|
|B/B||E/E or E/e||Black, no melanistic mask||Black|
|B/b*||Em/Em, Em/E or Em/e||Black, melanistic mask is not visible||Black|
|B/b*||E/E or E/e||Black, no melanistic mask||Black|
|b/b*||Em/Em, Em/E or Em/e||Brown/chocolate/liver, with melanistic mask||Brown|
|b/b*||E/E or E/e||Brown/chocolate/liver, no melanistic mask||Brown|
|> 2b||This dog carries more than two b-alleles. The colour of this dog can be brown or black.
Option 1: The dog is black. In this case it also carries one copy of the B-allele.
Option 2: The dog is brown. In this case it carries only b-alleles.
* Three variants (bs,bc and bd ) of the b-allele are known. Since all three variants result in the same effect, in the above scheme all variants are named b. (B/bc, B/bd and B/bs are in the above scheme B/b. bc/bc, bc/bd/bd/bd, bs/bc, bs/bd and bs/bs are in the above scheme b/b).