S-Locus (Piebald)

The white spotting patterns that occur in many dog breeds do not have a uniform genetic basis. The Microphthalmia Associated Transcription Factor gene (MITF gene) is associated with many white spotting patterns. This gene is also known as the S-Locus. There are three major white spotting patterns described. One pattern is called “Irish spotting” and is a symmetrical pattern with white markings on the undersides, collar and muzzle, and/or blaze as demonstrated by breeds such as the Boston Terrier, Corgi, Bernese Mountain dog and Basenji. Another pattern of less symmetrical white spotting in which random white spots occur on the body of the dog is often called piebald, parti or random white and is observed in several breeds, including the Beagle and Fox Terrier. The third major pattern is called extreme white and results in a dog that is almost entirely white but usually has at least some color on the head. Furthermore, there is a pattern called mantle, this pattern is similar to Irish spotting but with more white extending onto the thigh and up the torso, as seen in some Great Danes. Another pattern that is similar to Irish spotting is called flash or pseudo-Irish and occurs in Boxers. A mutation found in the MITF gene is associated with the piebald spotting pattern in more than 25 different dog breeds. The Coat Colour Piebald test (H326) tests for the genetic status of this mutation. It results in two variants (alleles). The allele N does not produce a piebald pattern, therefor dogs with two copies of the N allele do not display the piebald pattern. The allele S is associated with the piebald pattern, however the amount of white spotting expressed varies from breed to breed and among individuals within a breed. In many breeds such as Collie, Great Dane, Italian Greyhound, Shetland Sheepdog, Boxer and Bull Terrier, piebald behaves as a dosage-dependent trait. In those breeds the allele S is semi-dominant. One copy of the S allele (S/N) results in a limited white spotting pattern. Dogs with two copies of the  S allele (S/S) display more extreme white with colour only on the head and perhaps a body spot. In Boxers and Bull Terriers, dogs that have two copies of the S allele (S/S) are completely white while dogs that only have one copy of the S allele (N/S) display the mantle pattern (called flash in these breeds). However, additional mutations in MITF or other white-spotting genes that affect the amount of white being expressed appear to be present in these breeds. In some other breeds, the allele S is recessive and in those breeds two copies are needed to produce the piebald pattern.

The Coat Colour Piebald test encloses the following results: