Splashed white is a white spotting pattern characterised by extensive white markings on the head and legs. Sometimes the white can extend to the body. There are many mutations that cause the SW pattern.
Symbols
SW1, SW2, SW3, SW4, SW5, SW6
Breeds known to carry these mutations
Quarter Horses and related breeds, Paint Horses, Appaloosas, and many others. There are also horses showing SW patterning from many breeds that do not test positive for any of the known mutations.
Inheritance pattern
Autosomal dominant
Description
Splashed White is characterised by a large blaze, sometimes extending into a hood covering nearly the entire head, extensive white on the legs, and variable white on the body. The horse can look like it was dipped in white paint feet first. The eyes can be wholly or partially blue and the horse may be deaf. Deafness cannot be diagnosed just by looking at the distribution of white on the horse.
The mutations that cause all the known splashed white patterns have been identified in one of two genes.
SW1, SW3, SW5 and SW6 mutations are found on the MITF gene, while the mutations for SW2 & SW4 occur on the PAX3 gene.
Two copies of SW1 are often associated with a larger amount of white on the body than just one copy. The rest of the splashed whites seem to behave in a dominant fashion, so one copy has the same impact as two (in the very limited cases where homozygous horses have been found). Splashed white can sometimes combine with other white spotting mutations (such as sabino 1 or frame overo (OLWFS)), to significantly boost the amount of white on the horse.
Horses carrying SW5 appear to be at increased risk of deafness. It is thought that this might be because the SW5 mutation removes a large portion of the MITF gene, thereby impacting its function more significantly than mutations that just swap a small part of the DNA sequence.
The EGRC currently tests for 6 of the 10 known SW patterns.
Interpretation of results
n/n: The horse does not carry the mutation associated with SW1, SW2, SW3, SW4, SW4, SW5 or SW6.
SW1/n; SW2/n; SW3/n; SW4/n; SW5/n or SW6/n: The horse carries one copy of the notified SW mutation and likely has a blaze and white stockings. Horses carrying two copies of the SW1 mutation may also have more extensive white including on the belly.
SW1/SW1 or SW2/SW2: The horse carries two copies of the notified SW mutation. It will transmit that splashed white variant to all its offspring.
No horses have ever been identified carrying two copies of the other SW mutations we test for (SW3/SW3; SW4/SW4; SW5/SW5 and SW6/SW6), but it is not known if this is because these mutations are very rare, or if they are fatal to the developing embryo.
Horses have been identified carrying more than one type of SW variant, for example one copy each of SW1 and SW3 (SW1/SW3). Because the mutations causing splashed white are found on two different genes, horses can also have two copies of SW1 or one of SW3, SW5, SW6 (found on MITF), alongside two copies of SW2 or one of SW4 (found on PAX3).
References
Magdesian et al. A de novo MITF deletion explains a novel splashed white phenotype in an American Paint horse. Journal of Heredity 2020; 111:287-293. doi: 10.1093/jhered/esaa009
Henkel et al. Whole-genome sequencing reveals a large deletion in the MITF gene in horses with white spotted coat colour and increased risk of deafness. Animal Genetics 2019; 50:172-174. doi:10.1111/age.12762
Hauswirth et al. Novel variants in the KIT and PAX3 genes in horses with white-spotted coat colour phenotypes. Animal Genetics 2013; 44:763-5. doi: 10.1111/age.1205
Hauswirth et al. Mutations in MITF and PAX3 cause ‘‘Splashed White’’ and other white spotting phenotypes in horses. Plos Genetics 2012; 8:e1002653. doi: 10.1371/journal.pgen.1002653