Genetics of coat colors

I've been reading about the genetics of coat coloration in dogs recently. Not all of it is known, and there are some different interpretations out there. However I went by what was written on this papillon site and looked at Dragon's phenotype (the physical characteristics we can observe and measure) to have a guess at his genotype (the underlying genes which determine his coat colors).

(Least flattering photo ever. It looks like his eyes are pointing in different directions.)

Ahem. Locus S, determining the amount of white (non-pigmented) hair on the body: PIEBALD: The s^p or piebald allele is responsible for producing a coat which is about 50% white and 50% color. Papillons are affected by the s^p allele for piebald. Normally, the white areas are found on the chest, neck, legs, belly, around the loin area, and the tip of the tail. However, due to modifier genes, there is a lot of variation in the amount of white that an individual Papillon can have.

Locus A, determining the placement of eumalin (black pigment) in hair and restricting it to allow pheomelanin (red pigment) to show. Dragon is a tricolor. TRICOLOR: The fourth allele, a^t, is responsible for the tricolor. A tricolor has solid black body spots and gold markings which are more commonly referred to as "tan" points. The tan points are located over the eyes, a little on each cheek, some in the ears, and under the tail. The tan points range from shades of pale cream to deep red... A different variation of the hound tricolor is where the face retains the classic tricolor’s tan points, but gold or tan hairs are produced at the back of the head.

If tri-colored dogs have pigment underneath their tail, it will be brown and not black.

Dragon is mostly white underneath his tail, but there is a handful of brown hairs.

Locus S, which can dilute the pheomelanin (red) to a light cream or lemon shade. Dragon does not express the diluting genes. FULL PHEOMELANIN:  The [C-] genotype is the most common allele combination at the C locus. It allows for full pheomelanin (gold) pigment in the phenotype. Most Papillons are [CC] (a.k.a. [C-]) and therefore show regular gold coloring in their coat.

Locus E, which can create a mask, an all-red/yellow dog with no eumelanin (black), or have no effect. (The site listed above also says that this gene can create brindle, but other sites I've read (example here) say that that has been disproven since it was originally hypothesized.) EXTENTION: The E allele does occur in Papillons and is relatively simple to explain. If a Papillon is [E-], it will be whatever color the other genes express. The allele E does not produce any color of its own; it just allows the coat colors from the other series to show. It seems that regular E is recessive to E^m (which creates a mask), and masks are very common in papillons, but Dragon does not have one.

Locus G: The G locus is responsible for graying or frosting in Papillons. A dog who is [gg] at the G locus shows normal coloring throughout its life. It may grow some gray hairs when it is old, but this has nothing to do with the G series. In dogs who are [GG] or [Gg] some of the dark hairs are replaced by non-colored, or white, hairs. These new interspersed white hairs cause the coat to appear faded. This lightening may start immediately after birth or after several months, and it may stop after the dog has grown its adult coat or it may continue throughout the dog’s life. There is also great variability in how much graying will occur. So far it looks like Dragon does not have the graying gene.

Locus K: A dog who has at least one K allele will be black in color regardless of what other alleles the dog has on other loci. Clearly Dragon is kk: A who is homozygous recessive [kk] will not be black. His color will be affected by all the other alleles.

Locus T, for ticking. It is hard to photograph, but Dragon has black ticking all over the white portions of his back, brown ticking down his legs, and even a couple of brown spots on his muzzle. The T locus is responsible for ticking or freckling. The dominant allele, T, is responsible for producing spots of color on a white background. The amount of ticking varies greatly. On Papillons it usually occurs on the legs or muzzle. However, there have been Papillons who have exhibited ticking over all the white regions of their coat. Interestingly, ticking is not visible at birth. It usually demonstrates itself when the puppy several weeks of age.

I am continuing my education about the genetics of coat colors and am excited to compare what I've learned with the dogs that I see every day at work. Some are very easy to figure out (like Labrador Retrievers) while others still mystify me.