only effects Pheomelanin, but Eumelanin as well. This mutation does not effect all breeds the same. Pigment type Several loci can be grouped as controlling when and where on a dog eumelanin (blacks-browns) or phaeomelanin (reds-yellows) are produced: the Agouti (A), Extension (E) and Black (K) loci. Intercellular signaling pathways tell a melanocyte which type of melanin to produce. Time-dependent pigment switching can lead to the production of a single hair with bands of eumelanin and phaeomelanin. Spatial-dependent signaling results in parts of the body with different levels of each pigment. MC1R (the E locus) is a receptor on the surface of melanocytes. When active, it causes the melanocyte to synthesize eumelanin; when inactive, the melanocyte produces phaeomelanin instead. ASIP (the A locus) binds to and inactivates MC1R, thereby causing phaeomelanin synthesis. DEFB103 (the K locus) in turn prevents ASIP from inhibiting MC1R, thereby increasing eumelanin synthesis. A (agouti) locus The alleles at the A locus are related to the production of agouti signalling protein (ASIP) and determine whether an animal expresses an agouti appearance, and, by controlling the distribution of pigment in individual hairs, what type of agouti. There are four known alleles that occur at the A locus: Ay = Fawn or sable. Tan with black whiskers and varying amounts of black-tipped and/or all-black hairs dispersed throughout. Fawn typically referring to dogs with clearer tan and sable to those with more black shading. aw = Wild-type agouti. Each hair with 3-6 bands alternating black and tan. Also called wolf sable. at = Tan point. Black with tan patches on the face and underside - including saddle tan (tan with a black saddle or blanket). Phaeomelanin production is limited to tan points; dark portions of the dog are solid eumelanin hairs. a = Recessive black. Solid black, inhibition of phaeomelanin. ayt = Recombinant fawn (expresses a varied phenotype depending on the breed) has been identified in numerous Tibetan Spaniels and individuals in other breeds, including the Dingo. Its hierarchical position is not yet understood. Most texts suggest that the dominance hierarchy for the A locus alleles appears to be as follows: Ay > aw > at > a; however, research suggests the existence of pairwise dominance/recessiveness relationships in different families and not the existence of a single hierarchy in one family. Ay is incompletely dominant to at, so that heterozygous individuals have more black sabling, especially as puppies and