Dominant White (W)
Gene or Region: KIT
Reference Variant: Many, View Individual Whites
Mutant Variant: Many, View Individual Whites
Affected Breeds: Many, View Individual Whites
Research Confidence: High - Mutations in KIT have been well-documented to cause white spotting in both the horse and other species
Explanation of Results: Wx/Wx = homozygous for Dominant White, white markings expressed Wx/n = heterozygous for Dominant White, white markings expressed n/n = no variant detected
General Description
“Dominant white” refers to a class of white spotting patterns in which a horse with one copy will display markings on 50-100% of the body. While several of the W alleles produce “dark eyed whites,” there are a few dominant white individuals with blue or part-blue eyes. Often times, a white horse born from minimally marked/solid parents with no history of large amounts of white in the pedigree are the result of a new mutation in KIT. Studies in mice have indicated that many KIT mutations are embryonic lethal, such that any offspring with two mutant copies are reabsorbed early on in the pregnancy. In this horse, this effect would likely be seen as sub-fertility, though no study specific to the horse exists, and some mutations have well documented healthy “homozygous” (two copy) individuals.
** 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.
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.
More Horse Color Genetics
Champagne
Champagne (CH) is a dilution that affects all coat colors. Champagne foals are born with pink skin and blue eyes that slightly darken with age. Adult champagne horses have a distinct pumpkin colored skin with mottling in the hairless regions, as well as amber/green/tan eyes. Horses with multiple dilutions can be difficult to accurately identify color without genetic testing.
Congenital Stationary Night Blindness & Leopard Complex
Congential Stationary Night Blindness (CSNB) is characterized by the inability to see well in low light and no-light situations. It is linked to Leopard Complex Spotting (LP), where homozygous horses (LP/LP) will have CSNB. Congential Stationary Night Blindness is present at birth and is non-progressive.