Genetic basis of Black and Tan coat pattern - is primarily controlled by the A-locus (Agouti signaling protein gene, ASIP) and further modified by other loci. On the A-locus, the tan point allele (at) produces the classic black and tan markings: black coat with tan “points” (muzzle, eyebrows, legs, chest, underside). The Saddle tan pattern is a modifier of the tan point (at) allele, controlled by the As locus, involving the RALY gene. The RALY gene has two important variants: The WT (wild type) allele causes the saddle tan pattern, where black pigment recedes to a patch on the back resembling a "saddle." The dup (duplication) allele leads to the standard black and tan pattern. Saddle tan is a dominant modifier; dogs with at least one copy of the WT allele at the RALY locus will show the saddle tan pattern if the base tan points (at/at) and other pigment genes allow it. Expression of black and tan or saddle tan also depends on interaction with: E-locus (MC1R gene) for pigment production (at least one dominant E or Em allele), K-locus (CBD103 gene), where dogs need to be ky/ky (non-dominant black) to express A-locus effects fully.
Pathophysiology - The Agouti Signaling Protein (ASIP) influences the distribution of eumelanin (black/brown) and phaeomelanin (tan/red) pigments. The tan point allele (at) localizes black pigment to most of the body but produces tan markings on specific areas. The saddle tan modifier causes the black pigmentation to shrink mainly to the back (the “saddle”), with tan expanding over much of the rest of the body, especially the head, chest, neck, and legs. Puppies with saddle tan are usually born with a standard black and tan pattern, but black areas recede as the dog matures.
Complications - This is a coat color pattern, so no direct health complications arise from the specific black and tan or saddle tan genotypes. Some variability in shade and extent of black and tan markings can occur due to interactions with other pigment genes. Clearance of black pigment in saddle tan can sometimes overlap with other dilution or modifier genes, affecting coat appearance.
Why This Matters to Breeders and Vets - Understanding the genetic basis helps breeders predict coat color outcomes in litters, manage breed standards, and offer accurate genetic testing information. Distinguishing saddle tan from black and tan can be important for breed clubs and show ring conformity. For vets, recognizing the genetic background may assist in informing owners about coat color heritability and related breeding considerations.
Summary - The Black and Tan coat color pattern in dogs results from the interaction of the A-locus tan point allele (at), the K-locus (must be ky/ky), and E-locus alleles that allow eumelanin expression. The Saddle tan pattern is a dominant modifier involving the RALY gene (As locus), where the wild-type allele (WT) causes the black pigment to recede into a saddle-shaped patch on the back. Dogs with two copies of the duplicated allele (dup) at RALY present the classic black and tan pattern with more extensive black areas. Puppies with saddle tan usually resemble black and tan at birth, with black pigment receding as they mature. Genetic testing for these loci can accurately identify and predict coat color outcomes and assist breeders in making informed decisions.