Determines solid versus parti/piebald — the amount and placement of white on the coat, from a small chest spot to mostly white with colored patches.
Alleles
S: Solid / minimal white
Little to no white spotting. Dominant over sp. S/S dogs are solid-colored (or may have a small chest spot or toe white). S/sp dogs may show a little more white than S/S, as the dominance is incomplete.
sp: Piebald / parti
When two copies are present (sp/sp), produces large areas of white — the classic parti or piebald pattern. Amount of white varies from roughly 30% to nearly 90% white depending on modifier genes. Piebald is the same genotype as parti; the name difference is breed-specific.
What the S locus does
MITF (Microphthalmia-Associated Transcription Factor) influences how far melanocyte precursor cells migrate across the developing embryo. Less migration leaves white (unpigmented) areas where no melanocytes arrived to produce pigment.
The S locus acts with incomplete dominance: S/S dogs are solid or have minimal white; S/sp dogs are 'carriers' but may show a small chest spot or tuxedo markings; sp/sp dogs show the classic parti or piebald white pattern. Additional modifier loci influence the exact amount and placement of white even within sp/sp dogs.
Parti vs piebald — same genetics, different names
Parti (common in Poodles, Yorkies, doodle breeds) and piebald (common in French Bulldogs, Beagles, Basset Hounds) are the same genetic pattern — sp/sp at the S locus. The different names are breed community conventions, not different genotypes.
A parti Poodle, a pied French Bulldog, and a piebald Dachshund all carry the same sp/sp foundation, though breed-specific modifiers and the dog's base color (from E, K, A, B, D loci) determine what the colored portions look like.
Extreme white and health considerations
As the amount of white increases — especially when it covers the head, ears, and eyes — there is an increased risk of pigment-related deafness. This is because the same melanocytes that color the coat also develop the stria vascularis in the inner ear; without them, some dogs are deaf in one or both ears.
This risk is documented in Dalmatians, Bull Terriers, and some other heavily piebald breeds. In toy breeds with parti marking the risk is lower but not zero, especially in extreme-white dogs. BAER testing (Brainstem Auditory Evoked Response) can identify deafness in puppies before placement.
Responsible breeders who produce parti pups discuss the extreme-white risk transparently with families and test as needed.
Breeding for parti or piebald
Two sp/sp parents produce 100% sp/sp (parti) offspring. An sp/sp parent × an S/S parent produces 100% S/sp — all appearing solid or near-solid but all carrying parti. Two S/sp parents produce 25% sp/sp, 50% S/sp, 25% S/S.
Because S/sp dogs can look nearly solid, DNA testing is the only way to know whether a solid-looking dog carries the parti allele. This is especially important in lines where parti has been introduced once.
S Locus FAQ
What is a parti or piebald dog?
A parti or piebald dog is sp/sp at the S locus (MITF gene). The two copies of the piebald allele cause large areas of white on the coat — the colored areas retain whatever color the other loci produce. Parti Poodles, Parti Yorkies, Pied French Bulldogs, piebald Dachshunds, and tricolor Cavaliers are all sp/sp dogs in different breeds.
What is the difference between parti and piebald?
Genetically, there is no difference — both are sp/sp at the S locus. 'Parti' is the term used in Poodles, Yorkies, and doodle breeds; 'piebald' or 'pied' is used in French Bulldogs, Dachshunds, and other breeds. The pattern is the same: irregular areas of white and color.
Can you predict where the white patches will be on a parti puppy?
Not precisely. While sp/sp dogs all have large white areas, the exact distribution is controlled by modifier genes and developmental randomness — it is not fully predictable from the parents' appearance. Generally, white tends to appear on the underside, chest, face, and legs. The amount of white can range from ~30% to ~90% even among sp/sp littermates.
Can an S/sp dog (carrier) show white?
Yes. Because S and sp show incomplete dominance, S/sp dogs often show a small amount of white — a chest blaze, white toes, or tuxedo markings — more than S/S dogs. However, the large parti pattern only occurs with sp/sp. The exact amount of white on an S/sp carrier varies and is influenced by additional modifiers.
Is extreme white dangerous in dogs?
Dogs with very high amounts of white — especially covering the head, ears, and eyes — have a moderately elevated risk of congenital deafness due to the absence of cochlear melanocytes in the inner ear. This is documented in Dalmatians, Bull Terriers, and some other heavily piebald breeds. In toy-breed parti dogs the risk is lower but not zero. BAER testing can screen puppies for deafness before placement.
How do I breed for parti puppies reliably?
Pair two sp/sp parents for 100% parti offspring. Pair one sp/sp with an S/sp carrier for 50% parti, 50% carrier — all offspring carry at least one sp. Two S/sp carriers produce 25% parti, 50% carrier, 25% solid. DNA test both parents to know their S locus genotype — a solid-looking dog can be an S/sp carrier.
What is a parti Yorkie?
A parti Yorkie is a Yorkshire Terrier that is sp/sp at the S locus — producing the characteristic large patches of white along with the breed's blue-and-tan or gold coloring. Parti Yorkies were first recognized by AKC as a color variety; they have the same genetics as any other parti dog, and their non-white areas show the same color locus expression as standard Yorkies.
Can a tuxedo dog produce parti puppies?
A 'tuxedo' pattern in dogs — mostly dark with white chest and sometimes white feet — may indicate S/sp (carrier) status. If the tuxedo dog is S/sp and is bred to another S/sp dog, 25% of offspring will be sp/sp parti. Breeding a tuxedo dog to an sp/sp parti parent gives 50% parti offspring. DNA testing the tuxedo dog confirms whether the white is from S/sp or just from natural variation in S/S dogs.