how is this identifed?
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what is dominant white? A.k.a  white spotting -  group of genetically related coat color conditions in the horse, best known for producing an all-white coat.
wiki:24625092
Dominant white [SEP] Introduction
Dominant white or white spotting is a group of genetically related coat color conditions in the horse, best known for producing an all-white coat, but also able to produce various forms of white spotting and white markings, several of which are sometimes referred to as sabino. There are many different forms of dominant white or white spotting; they are labeled "W1" through "W27" and are associated with the "KIT" gene, along with Sabino 1. The white spotting produced can range from flashy white markings like those made by "W20", to patterns similar to Sabino 1, to a fully white or almost fully white horse.