Splashed White Overo
(SW-1, SW-2, SW-3, SW-4)
Many splashed white horses are characterized by a large blaze, extended white markings on legs, variable white spotting on belly, pink skin and often blue eyes. In other cases, the unpigmented areas are quite small and cannot be distinguished from horses with other more subtle depigmentation phenotypes.
Typically though most splashed white horses look as though they where dipped in white paint feet first. The head, legs and belly may be white, 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, including 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, 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 arose several hundred years ago before the foundation of the modern horse breeds. Breed distribution of SW-1 includes Quarter Horse, Paint, 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.
Research also 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 LWO, SW-3 may be homozygous lethal (foal dies if it inherits two copies of the mutation). It is therefore not advisable to breed two horses that carry SW-3 as the risk of a lethal foal could be 25% and should be avoided.
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.
Animal Genetics offers genetic testing for splashed white mutations including SW-1, SW-2, SW-3 and SW-4:
Horses that carry combinations of the splashed white mutations and or Tobiano, lethal white Overo and other color deletions can display extensive white patterning or be completely white.
|SW1/SW1||Homozygous||Horse has two copies of the SW-1 mutation|
|N/SW1||Heterozygous||Horse has one copy of the SW-1 mutation|
|N/N||Negative||No copies of SW-1 mutation|
|SW2/SW2||Homozygous||Horse has two copies of the SW-2 mutation|
|N/SW2||Heterozygous||Horse has one copy of the SW-2 mutation|
|N/N||Negative||No copies of SW-2 mutation|
|SW3/SW3||Homozygous||Possible homozygous lethal|
|N/SW3||Heterozygous||Horse has one copy of the SW-3 mutation|
|N/N||Negative||No copies of SW-3 mutation|
|SW4/SW4||Homozygous||Possible homozygous lethal but no current information available|
|N/SW4||Heterozygous||Horse has one copy of the SW-4 mutation|
|N/N||Negative||No copies of SW-4 mutation|
The primary research was conducted at the Institute of Genetics, Vetsuisse Faculty, University of Bern, Bern, Switzerland.conis.
Additional work is still in progress.
Additional information about the splashed white testing will be added.