Splashed white is a colour pattern where extensive white is seen on the head, legs, and sometimes extending up the body. There are many mutations that cause the SW pattern
SW1, SW2, SW3
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.
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. This cannot be diagnosed just by looking at the distribution of white on the horse.
SW1 and 3 are found on the same gene (MITF) and SW2 is found on a different gene (PAX3). Two copies of SW1 is often associated with a larger amount of white on the body than just one copy.
The EGRC currently tests for 3 of the 6 known SW patterns.
SW is sometimes also associated with deafness. It was originally thought that if the white pattern covered the ears the horse was deaf. However, this has been shown to be untrue and you cannot tell by looking at the distribution of white pattern on the horse whether the horse is deaf.
Interpretation of results
n/n: Horse does not carry the SW1 mutation.
SW1/n: Horse carries one copy of the SW1 mutation and likely has a blaze and white stockings.
SW1/SW1: Horse carries two copies of the SW1 mutation and likely has a blaze and white stockings. It may also have white on its belly. It will pass SW1 on to all its offspring.
n/n: Horse does not carry the SW2 mutation.
SW2/n: Horse carries one copy of the SW2 mutation and likely has a blaze and white stockings.
SW2/SW2: Horse carries two copies of the SW2 mutation and will pass SW2 on to all its offspring.
n/n: Horse does not carry the SW3 mutation.
SW3/n: Horse carries one copy of the SW3 mutation and likely has a blaze and white stockings.
SW3/SW3: No horse has been identified as carrying two copies of SW3. It is not known if this is because this is a rare DNA mutation or if two copies are incompatible with life.
It is possible for a horse to carry both SW1 and SW2. It may also be possible (theoretically) for a horse to carry SW1, SW2 and SW3 (because SW2 is on a different gene to SW1 and SW3) but this has not yet been identified.
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