UC Davis School of Veterinary Medicine Veterinary Genetics Laboratory
Splashed-White

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Splashed white is a variable white spotting pattern characterized primarily by large, broad blaze, extended white markings in legs, variable white spotting on belly, and often blue eyes. Some, but not all, splashed white horses are also deaf. Four mutations have been identified – SW-1, SW-2, SW-3 and SW4 – that cause splashed white phenotypes in horses. SW1 and SW3 are mutations in the MITF gene. SW2 and SW4 are mutations in the PAX3 gene. SW-1 is found in several breeds - Quarter Horse, Paint, Morgan Horse, Trakehner, Miniature Horse, Shetland Pony and Icelandic Horse – and may be present in other breeds as well. Horses homozygous for SW-1 (SW1/SW1) have been identified, which suggests that this mutation is not homozygous lethal. SW-2 and the rare SW-3 occur exclusively in certain lines of Quarter Horses and Paints. SW2 is currently considered not homozygous lethal based on a single SW2/SW2 horse. Horses that have SW2 may be deaf. Based on predictions from other species, SW-3 may be homozygous lethal but there is no confirmatory evidence. The current recommendation is that mating of two horses that carry SW-3 should be avoided. The rare SW-4 mutation has been identified in the Appaloosa breed and may cause a splashed white or a broad blaze.

Splashed white mutations are inherited as dominant traits with variable expression, which means that one copy of a SW mutation will produce a white spotting phenotype with variable amount of white. Horses that carry combinations of the splashed white mutations, tobiano or lethal white overo can display extensive white patterning or be white.

The VGL offers genetic tests for the 4 splashed white mutations SW-1, SW-2, SW-3 and SW-4.

Results are reported as:

Splashed White (SW1, SW3)
N/N No copies of SW1 or SW3 detected.
N/SW1 1 copy of SW1 detected.
SW1/SW1 2 copies of SW1 detected.
N/SW3 1 copy of SW3 detected.
SW3/SW3 2 copies of SW3 detected.
SW1/SW3 1 copy of SW1 and 1 copy of SW3 detected.

Splashed White (SW2, SW4)
N/N No copies of SW2 or SW4 detected.
N/SW2 1 copy of SW2 detected.
SW2/SW2 2 copies of SW2 detected.
N/SW4 1 copy of SW4 detected.
SW4/SW4 2 copies of SW4 detected.
SW2/SW4 1 copy of SW2 and 1 copy of SW4 detected.

References:

R Hauswirth, B Haase, M Blatter, SA Brooks, D Burger, C Drögemüller, V Gerber, D Henke, J Janda, R Jude, KG Magdesian, JM Matthews, P-A Poncet, V Svansson, T Tozaki, L Wilkinson-White, MCT Penedo, S Rieder and T Leeb. Mutations in MITF and PAX3 cause ‘‘Splashed White’’ and other white spotting phenotypes in horses. Plos Genetics 2012, 8(4) e1002653.

Hauswirth R1, Jude R, Haase B, Bellone RR, Archer S, Holl H, Brooks SA, Tozaki T, Penedo MC, Rieder S, Leeb T. Novel variants in the KIT and PAX3 genes in horses with white-spotted coat colour phenotypes. Anim Genet. 2013, 44 (6):763-765. doi: 10.1111/age.12057. Epub 2013 May 9.

 
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