|BROWN (B Locus Gene):
This coloration is often referred to as "Red". Genetically, this color is brown and
is the homozygous recessive allelic pair, "b/b". If in fact it was a true red, then the
allele would be non-extension "e".
The Brown Locus affects only eumelanin (affects only black/brown, not
red/yellow). 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 will 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 to amber,
gold, olive greenish-gold or an orange coloration.
Genes that code for color can easily be manipulated by the breeder to achieve a
certain color, or breed a certain color out of the bloodline.
In order to produce the brown color: both parents could be the brown/chocolate
color (b/b); or one of the parents must be the brown/chocolate color (b/b) -- with
the other parent carrying the brown/chocolate allele (B/b); or both parents (which
could be black in color) must be a carrier of the "B" gene in the heterozygous
RULES OF THUMB (when dealing with brown/chocolate colored dogs):
If you see a black puppy (with or without tan points and/or with or without white
spotting) in a litter where both parents are brown/chocolate ---- then that puppy
has a different sire. It is not genetically possible for two brown (b/b) dogs to
produce black (B/B or B/b) offspring.
If you DO produce a black puppy from two brown parents, there is no need to
panic. Contact Texas A&M College of Veterinary Medicine, Dr. Murphy's Lab and
tell them you need to verify parentage of your litter. You will need to test the
puppy in question, the dam and the suspected sire. If you have more than one
male that is any color other than brown, it is advisable to send all of their samples
in for testing. If you do not have any other males, other than the brown one, look
for a hole in your fence. AND, if you sent your bitch off to be bred, I'd contact the
breeder and mention this website.