Brown aka Chocolate
BB, Bb, bb
TYRP1 Gene. There are two alleles
1. B dominant full base color
2. b recessive brown
TYRP1 is a modifier of eumelanin, not a dilution gene as in blue (dd). The dilution gene affects how dense the pigment is, which makes it appear pale as if you mixed white into black paint making gray. The chocolate bb does not dilute, but changes the shape of the molecules of the pigment. The different shape reflects light in a different way and therefore changes the black to chocolate.
When you have a bb dog, black pigment is modified to chocolate. If your dog is red or yellow base, the bb does not modify the hair color, but does modify nose, eyerims, and foot pads from black to chocolate and eye color to amber or gold.
BB: Does not carry chocolate, full base color, cannot have chocolate offspring
Bb: Dog is base color and carries 1 copy of chocolate
bb: 2 copies of chocolate, full chocolate
The hair color on Chocolates can vary slightly, but, there is no getting around the LIVER Colored nose. The nose, eyeliner, and pads should be some variation of brown, whether Hershey type chocolate or a more diluted chocolate as in chocolate fawn/sable. If the nose is not liver (brown) it is not a true chocolate. Chocolate color in dogs is basically a modified Black dog, where black is modified to chocolate. For the Chocolate Gene to make a dark chocolate dog, the Full Black base has to be there.
The chocolate gene affects black pigment only. If a dog is bb, all of the black in the coat will be turned to chocolate. This includes all patterns. Chocolate turns the nose and eye color, usually to an amber, yellow, or gold color. The nose color is the best way to tell chocolate from a black or blue. When you combine a chocolate dog with a blue dog, bb + dd, you get a lilac, which is the color of a Weimaraner.
Chocolate is recessive, so it takes 2 copies to present on the dog. If a dog is full black base and is bb, all the black in its coat will become a chocolate. Bb and BB have no visible effect.
As if this were not confusing enough, I have now been advised that there are 3 Sequences to the b genotype. There are a number of different versions of the b allele, but all of them result in the same coat color. These variations mean you can have either b, b2, or b3 on your dog’s DNA. This can be on one or both sides of the complete gene. Therefore, it is possible to have your dog dna any one of the following types:
Bb Bb2 Bb3 bb bb2 bb3 b2b b2b2 b2b3 b3b b3b2 b3b3
Some labs recognize all 3 sequences and some don't, so you could have a b2 or b3 on your dog and not know it.