Dominant White 26 (W26)
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: W26/W26 = homozygous for Dominant White 26, trait expressed W26/n = heterozygous for Dominate White 26, trait expressed n/n = no variant detecte
General Description for Dominant White 26 (W26)
Dominant White 26 (W26) is found in Australian Thoroughbreds and may result in a partial to full white coat.
W26 Founder: Marbrowell, Australian Thoroughbred mare
W26 Phenotype: Partial to full white
Gene Information
The W26 variant is a single base pair deletion on KIT that produces a partial to completely white coat pattern. It is believed to have originated in the Australian Thoroughbred mare Marbrowell, born in 1997. Marbrowell produced a white filly named Maybe Maybelline, who also carries Lethal White Over. Maybe Maybelline has one blue eye and a completely white coat and has recently foaled a medicine hat palomino. W26 appears to have arisen on a W20 allele, meaning all W26 horses will also test positive for at least one copy of W20. To date, no horses have been identified carrying two copies of W26.
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
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.
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.