Coat color genetics part VII: S-locus (Spotting)

by Annika Farstad
© 2006

The S-locus is a pattern locus, controlling the distribution of white. In the pigmented areas of the coat the dog can be any color as determined by other genes. This locus only controls the distribution of white.

There are four alleles to this gene.The series alleles are listed in decreasing order of dominance;
(S) Solid
(si) Irish Spotting
(sp) Piebald spotting
(sw) Extreme White piebald spotting
All of the spotting genes are assumed to be affected by the action of modifiers, with + (plus) modifiers being generally understood to increase the amount of pigment (decrease white) while - (minus) modifiers being assumed to decrease the amount of pigment (increase white.)

Dogs that have the allels for spotting often have spots in skin in the areas left white by the spotting allel. Often these spots appear gradually, puppies are born with skin pigmented in identical fashion to the coat- then spots will begin to appear in the unpigmented areas of skin, very much like the ticking gene. Nevertheless it is different from the ticking gene, where the spots appear in the coat also, not only the skin.

(S) Solid: Self-colour with no white. This is usually the solid coloured dog. However, because of the modifiers it is possible for this allele to also express minor white markings (white toes, white tail tip, or a star or streak on the chest).

(si) Irish spotting: White on muzzle, forehead, chest, belly, feet.

(sp) Piebald spotting: This allows large patches of colour to be expressed. Piebald spotting is a pattern of spots which occur randomly anywhere on the body, including the torso. They are not of consistent size or location therefore. Dogs that have piebald spotting (sp) can have very few colored spots In general, it seems a piebald has more than 50% white, white often crosses the back, and the pattern gives the impression of fairly large colored spots on a white ground. or very many. Piebald and irish spotting seem to overlap in phenotype in one direction, while piebald and extreme white overlap in the other. Although Piebald is sometimes refered to as Harlequin, these era most liekly due to two different genes.

(sw) Extreme White Spotting: Little and others have suggested that there is also an allele, sw, which is the most recessive in the S series that causes "extreme white spotting". iIn an sw dog one would expect to see large amounts of white, with minimal, or no, areas of base color. However, the distinction between a dog with few piebald spots and a dog that is "extreme white" spotted is not that clearcut.