Dominant White 22 (W22)
Gene or Region: KIT
Reference Variant: C
Mutant Variant: DEL
Affected Breeds: Thoroughbred
Research Confidence: High - Mutations in KIT have been well-documented to cause white spotting in both the horse and other species
Explanation of Results: W22/W22 = homozygous for Dominant White 22, no reported horses testing homozygous; may be lethal recessive W22/n = heterozygous for Dominant White 22, significant white markings to possibly all white coat with dark eyes n/n = no variant detected
General Description for Dominant White 22
Dominant White 22 (W22) is found in Thoroughbreds and Paints and may result in a sabino-like to completely white coat.
_W22 Founder: Airdrie Apache
W22 Phenotype: Sabino-like to completely white
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.