General Description for Dominant White 20

Dominant White 20 (W20) is found in many breeds and may result in white markings.

W20 Founder: Unknown

W20 Phenotype: White markings - minimal to bold face / legs - thought to extend other whitesYe

Gene Information

KIT is a tyrosine kinase receptor that is vital for normal development. Mutations in other species have led to white spotting, anemia, sterility, and certain types of tumors. However, no negative health effects associated with dominant white have ever been documented in the horse. The various W alleles encompass a variety of mutations, all resulting in changes to the encoded protein.

The W20 variant is associated with bold face and leg markings and can significantly increase the amount of white when combined with other white patterns. Discovered in 2007, its effect on coat color was not recognized until 2013. When combined with one copy of W5 or W22, horses tend to be white or nearly all white. On its own, W20 tends to increase the amount of white. The mutation is a missense mutation (c.2045G>A; p.Arg682His) located on exon 14 of the KIT gene. W20 has been identified in numerous breeds, including the German Riding Pony, German Warmblood, Thoroughbred, Oldenburger, Welsh Pony, Quarter Horse, Paint Horse, Appaloosa, Noriker, Old-Tori, Gypsy Horse, Morgan Horse, Clydesdale Horse, Franches-Montagnes, Marwari Horse, South German Draft, Paso Peruano, Camarillo White Horse, and Hanoverian Horse.

References

Haase B et al., “Allelic heterogeneity at the equine KIT locus in dominant white (W) horses.” (2007) PLoS Genet. 3: e195.

Haase B et al., “Seven novel KIT mutations in horses with white coat colour phenotypes.” (2009) Anim Genet. 40: 623-9.

Holl H et al., “De novo mutation of KIT discovered as a result of a non-hereditary white coat colour pattern.” (2010) Anim Genet. 41: 196-8.

Haase B et al., “Five novel KIT mutations in horses with white coat colour phenotypes.” (2011) Anim Genet. 42: 337-9.

Hauswirth R et al., “Novel variants in the KIT and PAX3 genes in horses with white-spotted coat colour phenotypes.” (2013) Anim Genet. 44: 763-5.

Holl H et al., “A novel splice mutation within equine KIT and the W15 allele in the homozygous state lead to all white coat color phenotypes.” (2017) Anim Genet. DOI: 10.1111/age.12554

Durig N et al., “Whole genome sequencing reveals a novel deletion variant in the KIT gene in horses with white spotted coat colour phenotypes.” (2017) Anim Genet. In press.