As Locus (Saddle Tan)

Affected Genes: RALY
Inheritance: Dominant Modifier
Mutation: chr24:23252756-23252770 (canFam3): 16 bp duplication (dup CCCCAGGTCAGAGTTT)
Breed(s): All Breeds
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Testing Summary

The Paw Print Genetics As Locus (Saddle Tan) coat color test evaluates the RALY gene for the presence of the "As" variant that is associated with a "saddle tan" coat color. The saddle tan coat color is only present in dogs that have As/As or N/As genotypes AND are also expressing tan points at the A Locus (ky/ky and at/at or at/a). The As variant is a Dominant Modifier of the tan points coat color that is associated with producing a saddle tan coat color in dogs already expressing tan points.

The As Locus (Saddle Tan) coat color test reliably determines if a dog has one of the following genotypes:

As/As

This dog carries two copies of an As Allele which is found in dogs with a saddle tan coat color. However, this dog’s coat color is also dependent on the E, A, and K loci. Saddle tan is found only in dogs that are also E/E or E/e at the E locus, ky/ky at the K locus, and at/at or at/a at the A locus. This dog will pass one copy of As to 100% of its offspring and can produce saddle tan dogs.

Interpretation: Saddle tan/creeping tan

N/As

This dog carries one copy of an As allele and one copy of N, the wildtype sequence. This combination of alleles is also found in dogs with a saddle tan coat color. However, this dog’s coat color is also dependent on the E, A, and K loci. Saddle tan is found only in dogs that are also E/E or E/e at the E locus, ky/ky at the K locus, and at/at or at/a at the A locus. This dog will pass the N allele to 50% of its offspring and the As allele to 50% of its offspring, which can produce saddle tan dogs.

Interpretation: Saddle tan/creeping tan (non-saddle tan Carrier)

N/N

This dog carries two copies of the N allele, which is not associated with a saddle tan coat color. This dog’s coat color is also dependent on the E, A, and K genes, among others. This dog will pass N to 100% of its offspring.

Interpretation: No saddle tan/creeping tan

Testing Recommendations:

Because the appearance of saddle tan is dependent on the E, A and K loci, full interpretations of a dog’s genotype may not be possible in the absence of this information and owners should consider testing their dogs for E, A and K when testing for Saddle Tan.

Detailed Summary

The Agouti Locus impacts canine coat color by regulating the temporal and spatial expression of the ASIP gene which when activated causes a switch from the synthesis of eumelanin (black pigment) to phaeomelanin (yellow/red pigment) in the melanocyte of the canine epidermis and hair follicles. The Ay variant of ASIP inhibits the MC1R gene (E locus) causing a switch from eumelanin to phaeomelanin production resulting in a sable/fawn coat color. The aw variant of ASIP produces a "wolf" sable/gray coat color. Other variations of ASIP, such as the at and a gene variants, impact canine coat color by producing tricolor/black and tan or bicolor/solid black coat colors, respectively. The variants of the ASIP gene are expressed in a hierarchical manner with Ay being dominant to and expressed over aw, aw being dominant to and expressed over at, and at being dominant to and expressed over a. However, the dog’s coat color is also dependent on the dog’s genotypes at the E, K, and B genes among others. In addition, the As Locus (Saddle Tan) acts as a Dominant Modifier of black and tan by converting it into a "saddle tan" coat color. The saddle tan coat color is characterized by a progressive shrinking of the black or eumelanin pigment away from the outer extremities over the lifetime of the dog reducing the black coat color to a smaller "saddle" on the back of the dog. Due to the shrinking of the dark pigment and the progressive expansion of the tan points, this coat color is also referred to as "creeping tan". The saddle tan or creeping tan coat color occurs only in dogs that are expressing tan points (E/E or E/e at the E locus, ky/ky at the K locus, and at/at or at/a at the A locus). The As Allele is distinguished by the lack of a 16 base pair duplication within the RALY gene and having one or two copies is associated with saddle tan. Although all dogs with saddle tan coat colors have As/As or N/As genotypes, not all dogs with one or two copies of the As allele have a saddle tan coat color. This suggests that there are other yet to be identified genes or modifiers that cause saddle tan in some dogs.

Testing Tips

The Paw Print Genetics As Locus (Saddle Tan) coat color test evaluates the RALY gene for the presence of the “As" variant that is associated with a "saddle tan" coat color. The saddle tan coat color is only present in dogs that have As/As or N/As genotypes AND are also expressing tan points at the A Locus (ky/ky and at/at or at/a). The As variant is a Dominant Modifier of the tan points coat color that is associated with producing a saddle tan coat color in dogs already expressing tan points.

There may be other causes of this condition in dogs and a normal result does not exclude a different mutation in this gene or any other gene that may result in a similar genetic disease or trait.

References

  • Dreger DL, Parker HG, Ostrander EA, Schmutz SM. Identification of a mutation that is associated with the saddle tan and black-and-tan phenotypes in Basset Hounds and Pembroke Welsh Corgis. J Hered. 2013 May-Jun; 104(3):399-406. [PubMed: 23519866]
  • Dreger DL, Schmutz SM. A SINE insertion causes the black-and-tan and saddle tan phenotypes in domestic dogs. J Hered. 2011 Sep-Oct; 102 Suppl 1:S11-8. [PubMed: 21846741]