Saddle tan is a dog’s coat color, which is considered as a modifier of the tan point gene. Another modifier of the tan point is the creeping tan. All these coat colors are caused by presence of specific genes of the A locus. The saddle tan phenotype is present in a limited number of dog breeds, including terriers, scent hounds, and herding dogs. While most of dog breeds can show black-and-tan phenotype, saddle tan is exhibited in only a limited number of breeds, indicating that saddle tan phenotype occurred later in breed development. This knowledge sheds light on the development and relations of modern dog breeds, hinting at common ancestor between those breed types.
Tan point gene is located on A locus. A locus is also known as the Agouti locus. It affects distribution of both, eumelanin and phaeomelanin. The agouti series contains four alleles: ay (sable), aw (agouti/wolf grey), at (tan points), and a (recessive black). Hierarchically they are ordered: Ay > aw > at >a. It is important to know that the genes of A series can be expressed only when there is no dominant black allele K on the K locus. In the case of one dominant black allele, a dog can be genetically sable, tan points or agouti but that will never be expressed.
Saddle tan is modifier of tan points. Tan points (at) is the almost bottom gene in the agouti series gene, which means that in order for it to be expressed, the dog must be homozygous, containing two at alleles. The only gene under the tan points gene is the recessive black, which is very rare.
The saddle tan modifier causes the black (eumelanin) coat color on a black and tan dog to retreat to the dog’s back, while rest of coat will be red. The red coat will extend over the whole head, the front of the check and neck and the top of the legs, while black will remain only on the back, tail and the back of the neck. Saddle tan dogs are usually born as black and tan, but the black areas reduce as the dog ages.
The Agouti Signaling Protein (ASIP) gene product takes place in eumelanin and phaeomelanin expression regulation pathways, resulting in the production of phaeomelanin instead of eumelanin. As previously mentioned, four ASIP alleles have been identified in dogs to date, each producing a different pattern of phaeomelanin and eumelanin over the body of the dog. Depending on RALY gene (hnRNP associated with lethal yellow), tan points of saddle tan will be expressed in dog’s phenotype. In order for saddle tan to be expressed, two copies of the wildtype RALY gene without the duplication need to be present. The saddle tan is inherited as an autosomal recessive trait.
Dreger, D. L, Parker, H. G., Ostrander, E., A., Schmutz, S. M. (2013): Identification of a Mutation that Is Associated with the Saddle Tan and Black-and-Tan Phenotypes in Basset Hounds and Pembroke Welsh Corgis. Journal of Heredity 2013:104(3):399–406.