|Image is from Flickr.com under a creative commons license|
This handsome fellow is named Duke and he's a Labrador retriever. He looks like any old chocolate lab, right? Not quite! He's actually a tan pointed mismark, but the tan is so similar to the liver color that it could be fairly easily missed. In fact, in certain light this dog's points are nearly invisible. Anyway, here's his genotype:
Since he has the classic tan markings of a tan pointed dog, he must be atat tan pointed at the Agouti locus. I am suspicious that the Labrador breed is fixed for tan point under the other genes that produce their normally solid-colored coats. This is because when a mismark of this type does show up, it's tan pointed rather than expressing any of the other Agouti genes.
Duke must also be bb liver, since he is clearly not black. All chocolate Labradors are homozygous for the liver gene.
As for how he can possibly have tan points in the first place, most Labradors are KK (homozygous dominant black) at the Black locus, though occasionally dogs will be Kkbr (black carrying brindle) or Kk (black carrying non-black/brindle). As such, they can produce puppies that are kbrkbr (homozygous brindle), kbrk (brindle carrying non-black/brindle) or kk (non-black/brindle). These last three genotypes allow the genes on the Agouti locus to show through and would be what produces the tan pointed and brindle pointed mismarks that are occasionally seen in the breed. Since I see no evidence of stripes in Duke's tan, he must be kk non-black/brindle.
So, that's atat bb kk or liver and tan (aka chocolate and tan mismark).