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 likely expressed depending upon the based color and white variant, some may include blue eyes. *Some combinations may include growth, immunity, hearing, eye or other birth defects and may possibly be lethal. Check carefully prior to breeding horses to prevent unviable combinations. Wx/n = heterozygous for Dominant White, white markings to completely white coat likely expressed depending upon the base color, white variant and developmental factors. n/n = no variant detected
General Description of Dominant White
“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
IP/Brindle
Incontinentia Pigmenti (IP) is a brindle-like appearance found only in female horses. Affected male embryos die during development in utero. Ip foals develop pruritic, exudative lesions soon after booth. The affected regions progress into wart-like lesions and areas of alopecia (hair loss), with occasional wooly hair re-growth. Affected horses display streaks of darker and lighter coat coloration from birth.
Lethal White Overo
Lethal White Overo is a recessive trait characterized by patches of white bordered or “framed” by normal pigmentation. It is usually accompanied by blue eyes and generally white patterning does not cross the topline. Homozygous foals (LWO/LWO) are born completely white and with megacolon, an abnormal dilation of colon which necessitates immediate euthanasia.