White spotting (previously known as dominant white) is one of several known depigmentation phenotypes in horses. It is the result of a spontaneous mutation of the KIT-gene which is why white foals tend to be a surprise when born to solid parents. White spotting patterns are highly variable in expression. They can range from minimal markings—sometimes described as sabino-like spotting—to horses that appear almost entirely white. White spotting mutations themselves do not affect eye color. As a result, horses with white spotting patterns usually have brown eyes unless another gene influencing eye color is also present. For many years, the sabino-like patterning seen in some heavily marked Thoroughbred horses was believed to be caused by the sabino allele Sb1. However, genetic testing has shown that Sb1 has not been identified in Thoroughbreds, and the white patterning observed in the breed is instead associated with other mutations (W) of the KIT gene. White spotting should not be confused with Lethal White Syndrome, which is caused by a different gene and occurs in frame overo horses. Likewise, white horses are not albinos, as true albinism has not been documented in horses. White spotting mutations are generally inherited as autosomal dominant or incomplete/semi-dominant traits. They require only one copy of the gene for expression, though homozygous horses often display more white (incomplete dominance) or, in some cases, the mutation is lethal.

🪄 Founder: White Beauty (born 1963) 🔬 Genomic coordinate / type: chr3:79549540C>T / missense 🧬 Inheritance: autosomal dominant W2 usually produces a fully white individual though slight sabino-like patterning is also common. As homozygotes have not been observed, W2/W2 is thought to be embryonic lethal.