Splashed White - Inheritance and Prevalence

Inheritance and Prevalence

DNA tests exist for three forms of splashed white, labeled SW-1, SW-2 and SW-3. SW-1 is thought have originated hundreds of years ago and today has been identified in the American Quarter Horse, American Paint Horse, Icelandic horse, Miniature horse, Shetland pony and Trakehner. SW-2 and SW-3 appear at present to be confined to certain lines of Quarter horses and Paints, with SW-3 being particularly rare.

The splashed white pattern was first studied in Finnish Drafts and Welsh ponies by Klemola in 1933. Klemola's multiple studies on the topic have since been largely discredited, his text describing to detail a pattern now recognised as Sabino, and his illustrations featuring horses that according to modern knowledge would be a mixture of splashed whites, tobianos, sabinos, and combinations of multiple spotting factors. Originally believed to be very rare outside of Europe, splash is turning out to be more common than previously thought. The apparent spike in "new" splashed whites may be due to the pattern's tendency to masquerade as modest markings. For example, minimally marked splashed whites have been responsible for cropouts among American Quarter Horses.

Since the original study in the early part of the last century, in addition to the breeds identified as carrying the SW-1, -2 or -3 alleles, splashed white has also been identified, but not mapped, in American Saddlebreds, Morgan horses, the Irish Tinker or Gypsy horse, the Indian Kathiawari and feral Abaco Barbs of the Bahamas.

There may be additional forms not yet mapped. However studies suggest that it is not part of the linkage group that includes roan and tobiano. The KIT gene is not thought to be a candidate gene for splashed white. Preliminary studies from the early part of the 20th century suggested that splashed white was the result of a recessive gene, however this is no longer believed to be the case. It is more likely that many splashed whites go unidentified, whether because their markings are too minimal to register as "pinto" with breeders, confusion with sabino, or the confounding effect of multiple white spotting patterns. These effects also likely account for the apparent lack of homozygous splashed whites, as suspected homozygotes would still produce unremarkably marked offspring. It seems therefore more likely that splashed white is the result of an incomplete dominant gene, with homozygotes exhibiting the characteristic splashed white phenotype. Just as the presence of additional white patterning genes can increase the amount of white, other genes may be responsible for restricting the expression of splash.

In addition, other patterns have a similar look: in particular, some forms of sabino are called "splash" and vice-versa. The genetic mechanisms behind certain traits, such as blue eyes in a few Arabian horses with otherwise minimal markings, have yet to be identified.