Dominant White Mutations – W5, W10, W20, and W22

The KIT gene has crucial function for the development of many cell types, including blood and pigment cells (melanocytes). Mutations that affect normal functioning of the KIT protein often result in lack of melanocytes in the skin and hair follicles, which leads to white patterning in horses known as dominant white.

Dominant white patterns are variable ranging from extensive face and leg markings with or without minimal sabino like patterns, including roaning on the belly and/or belly spots, to an all-white horse. Eye color of dominant white horses is typically brown.

A number of different KIT mutations associated with white patterns have been identified in the horse. These include dominant white, Sabino-1, and Tobiano. To date, 28 of these mutations have been characterized as dominant white mutations. Dominant white mutations, with the exception of W20, arose recently and are restricted to specific lines within breeds.

W5 (c.2193delG) is found in descendants of the Thoroughbred stallion Puchilingui. W10 (c.1126_1129delGAAC) is found in descendants of the Quarter Horse stallion GQ Santana. W22 (Chr3:79548925-79550822del1898insTATAT) is found in descendants of the Thoroughbred stallion Airdrie Apache.

The majority of the dominant white mutations identified are thought to be detrimental to KIT protein function. When horses have one copy of these mutations they have a reduced number of melanocytes and thus have a white spotting pattern. However, when they have two copies of the mutation (homozygous), it is believed that reduction or absence of KIT signaling occurs, affecting more than just the pigment cells. For example, horses homozygous W5, W10, and W22 are likely embryonic lethal. However, this remains to be confirmed. It is unknown if horses that are compound heterozygous (two different mutations in the same gene, i.e. W5/W10, W5/22, or W10/W22) are viable, and testing through the VGL has yet to identify horses with these genotypes.

W20 (c.2045G>A) is a much older mutation and is found in many breeds. This mutation is thought to have a more minor effect on protein function as well as a subtler effect on the amount of white expressed unless in combination with other dominant white alleles (and perhaps other white spotting genes). In combination with other white pattern alleles, W20 has been shown to increase the amount of white patterning, producing an all white or nearly all white phenotype. Unlike W5, W10 and W22, the homozygous condition W20/W20 is not lethal.

W22 occurs on the W20 background, that means that all horses with the W22 mutation also have the W20 mutation. Since the W22 mutation has a greater impact on protein function than W20, the reported allele is W22. It is possible for horses to be compound heterozygotes, W20/W22, and these have been reported to have an all-white phenotype.

The Veterinary Genetics Laboratory offers tests for W5, W10, W20, and W22 mutations to owners who want to breed horses for dominant white or to determine the genetic status of horses with white patterning.