There is no single basis for white spotted patterns that occur in animals like cats, dogs and horses. In horses random white spotting, or deletions of color, has been determined to be caused by more than half a dozen known genetic factors. In more than 25 different dog breeds, a mutation found in a gene called the Microphthalmia Associated Transcription Factor-(MITF) is associated with a piebald spotting.
In many breeds, piebald behaves as a “dosage”-dependent trait. This means that the amount of white a dog expresses depends on how many copies of the S-Locus allele a dog receives. Dogs like the French Bulldog who have a single copy of the MITF variant (S locus allele) will express a limited white spotting pattern. Dogs that have 2 copies (S/S) of the variant will exhibit more white with very little color. In some breeds, dogs that are coded as S/S are completely white while dogs that are n/S have what is referred to as the mantle. The mantle coloration is seen in several breeds of dogs, but is perhaps best exemplified by the Great Dane (black with a white chest/throat and feet).
As with horses and other animals, dog breeds with white spotting patterns can be affected by additional mutations in the S-Locus and/or other genes that effect coat color deletion and distribution. For example, the Irish spotting pattern (similar to mantle) seen in many breeds of dogs like Collies, Shelties, or others is not caused by this mutation.
Karlsson EK, Baranowska I, Wade CM, Salmon Hillbertz NH, Zody MC, Anderson N, Biagi TM, Patterson N, Pielberg GR, Kulbokas EJ 3rd, Comstock KE, Keller ET, Mesirov JP, von Euler H, Kampe O, Hedhammar A, Lander ES, Andersson G, Andersson L, Londblad-Toh K. Efficient mapping of mendelian traits in dogs through genome-wide association. Nat Genet. 2007 Nov; 39(11):1321-8. [PubMed: 17906626]
|S/S||Homozygous: Dog has two copies of S-Locus resulting in a nearly solid white, parti, or piebald coat color.|
|n/S||Heterozygous: Dog has one copy of S-Locus. Results vary according to breed, with some limited white spotting in some breeds.|
|n/n||Negative: Dog is negative for the S-Locus. No white spotting will be present.|