BLACK or BROWN (CHOCOLATE) - B GENE LOCUS: (pigment color)

This gene, when in the homozygous recessive form, has a lightening effect on eumelanin (black-based colors) only. It has no effect on phaeomelanin (red-based colors).

It is believed that the Brown Locus codes for an enzyme, tyrosinase-related protein 1 (TYRP1), which catalyzes the final step in eumelanin production, changing the final intermediate brown pigment (dihydroxyindole) to black pigment.  SO, ALL dogs start as BROWN and after the final step --- this directs the color to be black.

When brown (b/b) is expressed, it means that the final step in eumelanin production has not been completed and the pigment remains brown. The brown color is not a genetic defect.

When the alleles are in the homozygous or heterozygous dominant form of B/B or B/b, the color and pigment (nose, eye rims and lips) remains (or directs the color to be) black.

When the alleles are in the homozygous recessive form (b/b), the color and pigment will be brown. This just means that the final step in eumelanin production of changing brown to black did not occur. Phaemelanin (yellow/red [e/e]) is not affected.  BUT, in the e/e colored dog, if the dog is also b/b; they'll be either red or yellow and will have brown pigment (nose, eye rims and lips). The pigment
granules produced by "bb" are smaller, rounder in shape, and appear lighter than pigment granules in "B" dogs. The iris of the eye is also lightened. Their are many shades of brown, from dark to light as shown in the photos below.

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