B^B (black, carrying black)
B^b (black, carrying brown)
This dog is genotypically a black dog (base color), as evidenced by the color
of his nose leather, eye rims and lips and of course his glossy black coat
color. He is also Dominant K, because he is masking the alleles from the
Agouti Locus. In order to know what he's carrying (color wise), or could
possibly produce, a person would need to know the color of his parents.
He is irish spotted and is non-white factored. You may have heard that a dog
of his color would be an excellent stud prospect to breed to a bitch of the
merle pattern, this is not always the case. Even though a dog of this color is
minimally white, one would still need to be selective because he could be
carrying the color for extreme pie bald -- and when bred to a merle, or one
that is pie bald or carrying pie or extreme pie bald, could produce offspring
with too much white. This is why it's very important to always know the
color of the parents and grandparents. If the lineage is not known, don't
breed from that dog.
NOTE: This dog is from verifiable lineage; all parents and ancestors are known.
The Extension Locus - E
This refers to the extension of eumelanin over the dog's body. The dominant form, "E", is normal
extension. The recessive form, "e", is non-extension. When a dog is homozygous for non-extension
(e/e), its coat will be entirely red/yellow (phaeomelanin based). All dogs that have a brown (chocolate)
coat will have at least one "E" allele, because of the production of eumelanin.
The way to tell the difference between an Agouti red/yellow and an Extension (e/e) red/yellow dog -- is
the Agouti red/yellow almost always have some black/brown hair in the coat (usually around the ears and
tail) and the Extension (e/e) dog won't. Another way is the Agouti red/yellow must have at least one
("A^y") allele and can carry at most one other agouti allele, the Extension (e/e) can carry any two Agouti
alleles (not necessarily "A^y").
DOMINANT BLACK -- "K"
The dominant form of black: completely dominates all formation of phaeomelanin pigment. In the past,
dominant black had been placed at the head of the Agouti series (symbol "A^s"). Now, it has been proven
to be part of a separate series, the "K" series, and not at the Agouti locus at all.
Dominant black (K) is epistatic to whatever is found at the Agouti locus (simply means that it causes the
Agouti allele to act differently from what it normally would), however; "e/e" is dominant to "K" at the E
When "K" is in the dominant form, "K/K" or "K/k", there would be no expression from the A Locus and the
color is dependant on what is at the E Locus.
When "K" is in the homozygous recessive form "k/k", the coat color will depend on what is located on
the "E" and "A" Locus.
BLACK/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 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.
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