Many splashed white horses are characterized by a large blaze, extended white markings on legs and variable white spotting on the belly, pink skin, and, often, blue eyes. In other cases, the unpigmented areas are quite small and cannot be distinguished from horses with other depigmentation phenotypes. Typically, most splashed white horses look as though they were dipped in white paint, feet first. The head, legs, and belly can be white, and are sometimes connected to a patch running up either side of the thorax. The margins of the white markings are crisp and well defined.
Occasional congenital deafness is associated with white or white-patterned, blue-eyed animals. This includes dogs and cats. It is important to note that most splashed white horses are not deaf. Hearing loss is due to the death of the necessary hair cells in the ear. This is caused by the absence of melanocytes in the inner ear. Although the majority of splash horses have pigment around the outside of the ear, the pigment must occur in the inner ear to prevent hearing loss.
Research suggests that the SW-1 allele (Splash White-1 allele) arose several hundred years ago, before the foundation of the modern horse breeds. This gene can appear in Quarter Horses, Paints, Trakehners, Miniature Horses, Shetland Ponies, and Icelandic Horses. It may be present in other breeds as well. Horses homozygous for SW-1 (SW1/SW1) have been identified. This suggests that this mutation is not homozygous lethal. If the color pattern was homozygous lethal, this means that a horse with two copies of the gene would die. Fortunately, this is not the case for SW-1.
Research suggests that SW-2 and the rare SW-3 may only occur in certain lines of Quarter Horses and Paints. It is also speculated that as in lethal white overo (LWO), SW-3 may be homozygous lethal (where the foal dies if it inherits two copies of the mutation). It is, therefore, not advisable to breed two horses that carry SW-3. If two carriers are bred, there is a 25% chance per foal born that it will inherit two copies and will develop the lethal symptoms.
SW-4 is a very rare form of splash white found in Appaloosas. The exact result of SW-4 is described as causing some form of splashed white or, in some cases, a broad blaze.
|Homozygous: Horse has two copies of the Splashed White 1 (SW1) mutation and will exhibit the Splashed White pattern in varying degrees. Horse will pass this gene on to all offspring.
|Heterozygous: Horse has one copy of the Splashed White 1 (SW1) mutation and will exhibit the Splashed White pattern in varying degrees. Horse can pass this gene on to any offspring.
|Negative: Horse is negative for the Splashed White 1 (SW1) mutation.
Mutations in MITF and PAX3 cause "splashed white" and other white spotting phenotypes in horses. Hauswirth R, Haase B, Blatter M, Brooks SA, Burger D, Drögemüller C, Gerber V, Henke D, Janda J, Jude R, Magdesian KG, Matthews JM, Poncet PA, Svansson V, Tozaki T, Wilkinson-White L, Penedo MC, Rieder S, Leeb T. PLoS Genet. 2012;8(4):e1002653. doi: 10.1371/journal.pgen.1002653. Epub 2012 Apr 12. Erratum in: PLoS Genet. 2019 Aug 2;15(8):e1008321. [PMCID:PMC3325211]