S Locus: Piebald, Parti, and White Spotting Genetics in Dogs

White spotting in dogs — from a small chest blaze to a fully parti or piebald dog — is primarily controlled by the S locus (MITF gene). Understanding S locus genetics helps breeders predict how much white their puppies will carry and avoids doubling up on extreme white, which can cause deafness and eye defects in some breeds.

The S Locus Alleles

S (solid / no white spotting): Dogs with two copies of S show no white spotting from this locus. Any white they carry comes from other genes or random somatic events.

SP (piebald): The SP allele causes white spotting. The amount of white increases with the number of SP copies — one copy (S/SP) typically produces a small amount of white (flash, blaze, collar, or socks), while two copies (SP/SP) produces a heavily white or piebald dog.

Some breeders and geneticists also note that there may be additional modifying alleles at this locus, and the degree of white expression can vary significantly even within the same genotype due to modifier genes.

Parti vs Piebald

"Parti" is the breeder term used in Poodles and Doodles. "Piebald" is used in Dachshunds, Beagles, and Bulldogs. Genetically, both refer to the SP/SP genotype at the S locus. The visual result — large patches of white on a coloured base — is the same regardless of what breeders call it.

Extreme White and Health Risks

In breeds where double merle (M/M) and extreme white (SP/SP) can occur together — such as Australian Shepherds, Collies, and some Dachshunds — the combination dramatically increases the risk of deafness and blindness. White areas lack pigment cells (melanocytes) in the inner ear and eye, which are needed for normal sensory function.

Even without merle, SP/SP dogs in breeds like Bull Terriers and Dalmatians have elevated rates of congenital deafness. BAER (Brainstem Auditory Evoked Response) testing is essential for breeding programs involving extreme white.

Breeds Where S Locus Is Especially Relevant

• Poodles: Parti (SP/SP) is a popular colour pattern
• Dachshunds: Piebald is a recognised pattern; double piebald increases health risks
• Beagles: Tri-colour and two-colour patterns involve S locus
• French Bulldogs: Pied (piebald) pattern is very popular
• Australian Shepherds: White factoring is common; breeders track S locus carefully
• Border Collies: Extensive white is S locus-driven
• Bull Terriers: All-white dogs are SP/SP

Predicting Puppy White Markings

If both parents are S/SP (minimal white), expect roughly 25 percent of puppies to be SP/SP (heavy white), 50 percent to be S/SP (moderate white), and 25 percent to be S/S (no white from this locus). Testing both parents before breeding lets you know exactly what you are working with.