Dominant White 27 (W27)
Gene or Region: KIT
Reference Variant: A
Mutant Variant: C
Affected Breeds: Australian Thoroughbreds
Research Confidence: High - Findings reproduced in multiple studies
Explanation of Results: W27/W27 = homozygous for Dominant White 27, trait expressed W27/n = heterozygous for Dominate White 27, trait expressed n/n = no variant detected
General Description for Dominant White 27 (W27)
Dominant White 27 (W27) is found in Australian Thoroughbreds and may result in a partial to full white coat.
W27 Founder: Milady Fair, Australian Thoroughbred mare
W27 Phenotype: Partial to full white
Gene Information
The W27 variant is a missense mutation of the KIT gene that produces a partial to completely white coat pattern. It is thought to have originated with the Australian Thoroughbred mare Milady Fair, born in 1960. Milady Fair produced a white filly named Glacial, who in turn had a white colt named Khaleben. Khaleben sired three more white offspring, the most prolific of which was Colorful Gambler. Most horses carrying this mutation trace back to her great-grand-colt, Colorful Gambler, known for his extensive sabino-like pattern. Through his descendants, W27 spread into show jumpers and the Paint breed, in addition to Thoroughbreds. W27 horses that are not completely white often display striking spotted patterns.
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. Negative health effects associated with dominant white have not been documented in the horse, although some horse owners have reported health issues in some homozygotes of certain Ws while some Ws are homozygous lethal. 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.