S Locus – Piebald, White Spotting Dog Coat Color
The white spotting locus, or S locus, is responsible for determination of most of white spotting dog coat color. Term ”white spots” actually represents white areas on the coat. In 1957., Little hypothesized that there is a single locus for white spotting dog coat color. Today this locus is known and it is referred to as MITF gene (Microphthalmia Associated Transcription Factor).
Color in dog’s coat is comes from two basic pigments: eumelanin, which is the basic black pigment, and phaeomelanin, which is the red pigment. Both pigments car vary in their intensity and color shade, depending on the influence of specific genes on specific loci. White dog coat color is not caused by any pigment, but by lack of pigment. When cells are not able to produce any pigment it is resulting in white coat color. MITF gene (S locus) is a key regulator that is controlling the pigmentation in dog’s coat. The MITF gene is involved in many developmental processes. It is involved in differentiation of neural crest-derived melanocytes, optic cup-derived retinal pigment epithelium and bone marrow-derived mast cells and osteoclasts and is a regulator that controls pigmentation in dogs.
At this locus, two main alleles have been described, S allele (no white, solid) and s (piebald). Another two alleles are known, which are most likely on another loci: sw (the extreme white), and si (phenotype known as Irish spotting).
Piebald varies in display of white spotting from limited to extensive, with often colored head and patches on the body. Piebald phenotype is common in Beagle and Fox Terrier breed.
Characteristic of Irish spotting is modest white spotting dog coat color, most often present as white collar and a white belly and legs, present most often in Bernese Mountain dog and Basenji.
Genotype Ssw causes phenotype known as flash and it is similar to Irish spotting. For this reason it is often referred to as pseudo-Irish.
Among dogs with extremely white phenotype (swsw genotype) deafness has been recorded, where 2% of the dogs appeared with bilateral deafness and 18% are unilaterally deaf.
Until now, several mutations in MITF have been identified. Mutations in the MITF have an impact on development and function of melanocytes in the skin, eye and inner ear. Some mutations cause reduction of the eye, known as microphtalmia, and therefore, have an effect on vision. On other hand, some MITF mutations cause an early onset of hearing disorders.
A mutation in the MITF causing piebald coat color has been identified in more than 25 different dog breeds. Piebald gene is recessive to dominant S (non-white) gene. This means if two piebald carrier dogs are mated ( Ss genotype, non-white phenotype), there is a 25% chance of a piebald cub occurring in the litter, and 50% that the puppy will be a piebald carrier. Although allele S is dominant over allele s, in many breeds genotype Ss shows co-dominant expression, which results in limited white spotting. As previously mentioned, there are several other types of white spotting such as Irish white spotting, but their DNA variations are not known.
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Baranowska Korberg I, Sundstrom E, Meadows JRS, Rosengren Pielberg G, Gustafson U, et al. (2014) A Simple Repeat Polymorphism in the MITF-M Promoter Is a Key Regulator of White Spotting in Dogs. PLoS ONE 9(8): e104363.