Dominant white

Dominant white (W)^[1][2] is a group of genetically related coat color alleles on the KIT gene of the horse, best known for producing an all-white coat, but also able to produce various forms of white spotting, as well as bold white markings. Prior to the discovery of the W allelic series, many of these patterns were described by the term sabino, which is still used by some breed registries.

White-colored horses are born with unpigmented pink skin and white hair, usually with dark eyes. Under normal conditions, at least one parent must be dominant white to produce dominant white offspring. However, most of the currently-known alleles can be linked to a documented spontaneous mutation that began with a single ancestor born of non-dominant white parents. Horses that exhibit white spotting will have pink skin under the white markings, but usually have dark skin beneath any dark hair.

There are many different alleles that produce dominant white or white spotting; as of 2022^[update] they are labeled W1 through W28 and W30 through W35, plus the first W allele discovered was named Sabino 1 (SB-1) instead of W1.^[3][4][5] They are associated with the KIT gene.^[B] The white spotting produced can range from white markings like those made by W20, to the irregularly-shaped or roaning patterns previously described as Sabino, to a fully white or almost fully white horse.

For many of the W alleles, the white coats are, as the name suggests, inherited dominantly,^[D] meaning that a horse only needs one copy of the allele to have a white or white spotted coat. In fact, some such alleles may be embryonic lethal when homozygous. Others, such as SB-1 and W20, are incomplete dominants, capable of producing viable offspring with two copies of the gene, and who generally have more white than horses with only one copy. In addition, different alleles which on their own give a white-spotted but not completely white horse, such as W5 and W10, can combine to make a horse completely white.

White can occur in any breed, and has been studied in many different breeds. Because of the wide range of patterns produced, some suggest the family be called “white spotting” rather than “white.” Other researchers suggest the term "dominant white" be used only for the W alleles thought to be embryonic lethal when homozygous.^[6]

White is both genetically and visually distinct from gray and cremello. Dominant white is not the same as lethal white syndrome, nor are white horses "albinos"—Tyrosinase negative albinism has never been documented in horses.