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

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Splashed white is a variable white spotting pattern characterized primarily by extremely large 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. 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.

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:

N/N No copies of SW-1 mutation
N/SW1 Horse has one copy of the SW-1 mutation
SW1/SW1 Horse has two copies of the SW-1 mutation
N/SW2 Horse has one copy of the SW-2 mutation
SW2/SW2 Horse has two copies of the SW-2 mutation
N/SW3 Horse has one copy of the SW-3 mutation
N/SW4 Horse has one copy of the SW-4 mutation

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.

 
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