"(As) - DOMINANT BLACK: This allele produces uniform coverage of dark pigment over the entire body. Its action is expressed in all dogs with black or brown coats. The (As) allele is almost completely dominant over others in the A series. The black color ranges from pure black to a black with a brownish cast (seal). Geneticists are uncertain if the allele is incapable to produce pure black without additional help from another locus, or if the brown cast indicates a heterozygous allele."

... "The red nose in our breed is produced from the Locus B pair with the homozygous recessive (bb) genotype in combination with the (Ay-) genotype."

"Question # 3: Where does the chocolate color come from?
The chocolate color comes from the Locus B pair with the homozyous recessive (bb) in combination with the (As) allele. The dilution gene (dd) will also modify this dark chocolate to a light or almost milk chocolate. In most cases, the nose color is also light brown or as we say ‘chocolate/red nose’."


http://www.adbadog.com/p_pdetails.asp?fspid=101