Dog Coat Color – S Locus (Piebald, White Spotting)

Acronym:	S Locus
Gene:	MITF
Mutation:	SINE insertion
Inheritance:	Autosomal co-dominant
Sample type:	CHS (Cheek Swab), WBE (Whole Blood EDTA)

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Genetics and characteristics

The white spotting locus, or S locus, is responsible for the determination of most white spotting dog coat color. The 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 the MITF gene (Microphthalmia Associated Transcription Factor).

Color in a dog’s coat comes from two basic pigments: eumelanin, which is the basic black pigment, and phaeomelanin, which is the red pigment. Both pigments car vary in 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 a dog’s coat. The MITF gene is involved in many developmental processes.  It is involved in the 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, the S allele (no white, solid) and s (piebald). Another two alleles are known, which are most likely on other loci: SW (the extreme white), and si (phenotype known as Irish spotting). Piebald varies in a display of white spotting from limited to extensive, with often colored heads and patches on the body. Piebald phenotype is common in Beagle and Fox Terrier breed. The characteristic of Irish spotting is modest white spotting dog coat color, most often present as a white collar and a white belly and legs, present most often in Bernese Mountain dogs and Basenji. Genotype Ssw causes a 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 the 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 the development and function of melanocytes in the skin, eye, and inner ear. Some mutations cause a reduction of the eye, known as microphthalmia, 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. The piebald gene is recessive to the 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 a 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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Results Reported As

 

 

 

 

 

 

 

 

 

 

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References:

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.

Schmutz, S., M., (2009.): MITF and White Spotting in Dogs: A Population Study. J Hered 100 (suppl 1):S66-S74.

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.