Dominant White 1No21 (W1No21)

“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.

W20 appears to work as a “white booster” when present with other spotting variants. Horses that inherit one copy of W20 and one copy of a different W allele are generally all white. Preliminary observations in a small number of horses suggests that W20 affects white spotting genes other than KIT as well.

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Genotype and Phenotype (Variant Details)

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W1

Reference allele: C (N)

Mutant allele: G (W1)

Affected Breeds: Franches-Montagnes

Founder: Cigale (born 1957)

Phenotype: mostly white, may be born with pigmentation along the topline that is lost with age

W2

Reference allele: C (N)

Mutant allele: T (W2)

Affected Breeds: Thoroughbred

Founder: Ky. Colonel (born 1946)

Phenotype: completely white

W3

Reference allele: T (N)

Mutant allele: A (W3)

Affected Breeds: Arabian

Founder: R Khasper (born 1996)

Phenotype: mostly white

W4

Reference allele: C (N)

Mutant allele: T (W4)

Affected Breeds: Camarillo White Horse

Founder: Sultan (born 1912)

Phenotype: mostly white

W5

Reference allele: C (N)

Mutant allele: - (W5)

Affected Breeds: Thoroughbred

Founder:  Puchilingui (born 1984)

Phenotype: sabino-like to completely white

W6

Reference allele: C (N)

Mutant allele: T (W6)

Affected Breeds: Thoroughbred

Founder: not stated (born 2004)

Phenotype: mostly white

W7

Reference allele: C (N)

Mutant allele: G (W7)

Affected Breeds: Thoroughbred

Founder: not stated (born 2005)

Phenotype: mostly white

W8

Reference allele: C (N)

Mutant allele: T (W8)

Affected Breeds: Icelandic Horse

Founder: unknown

Phenotype: mottled white

W9

Reference allele: C (N)

Mutant allele: T (W9)

Affected Breeds: Holstein

Founder: not stated (born 2006)

Phenotype: completely white

W10

Reference allele: GTTC (N)

Mutant allele: - (W10)

Affected Breeds: Quarter Horse

Founder: GQ Santana (born 2000)

Phenotype: sabino-like to completely white

W11

Reference allele: C (N)

Mutant allele: T (W11)

Affected Breeds: South German Draft

Founder: not stated (born 1997)

Phenotype: completely white

W12

Reference allele: TCTGC (N)

Mutant allele: - (W12)

Affected Breeds: Thoroughbred

Founder: not stated (born 2010, deceased 2010)

Phenotype: roughly 50% white

W13

Reference allele: C (N)

Mutant allele: G (W13)

Affected Breeds: Quarter Horse / American White Horse?

Founder: unknown

Phenotype: completely white

* This variant was originally described in a Quarter Horse / Peruvian Paso cross, and was assumed to have originated from the Quarter Horse side. However, we have observed W13 in the offspring of an American White Horse without known Quarter Horse ancestry.

W14

Reference allele: 54 bp present (N)

Mutant allele: 54 bp absent (W14)

Affected Breeds: Thoroughbred

Founder: not stated (born 1996)

Phenotype: completely white

W15

Reference allele: A (N)

Mutant allele: G (W15)

Affected Breeds: Arabian

Founder: Khartoon Klassic (born 1996)

Phenotype: partially white (heterozygous), completely white (homozygous)

W16

Reference allele: T (N)

Mutant allele: A (W4)

Affected Breeds: Oldenburger

Founder: not stated (born 2003)

Phenotype: mostly white

W17

Reference allele: T,A (N)

Mutant allele: A,G (W4)

Affected Breeds: Japanese Draft

Founder: not stated (born 2010)

Phenotype: completely white with one blue eye

W18

Reference allele: C (N)

Mutant allele: T (W18)

Affected Breeds: Swiss Warmblood

Founder: not stated

Phenotype: sabino-like

W19

Reference allele: T (N)

Mutant allele: C (W19)

Affected Breeds: Arabian

Founder: not stated

Phenotype: bald face, extended white on legs, belly spots

W20

Reference allele: C (N)

Mutant allele: T (W20)

Affected Breeds: Many

Founder: unknown

Phenotype: white markings

W21

Reference allele: G (N)

Mutant allele: - (W21)

Affected Breeds: Icelandic Horse

Founder: not stated

Phenotype: sabino-like with blue eyes

W22

Reference allele: C

Mutant allele: DEL

Affected Breeds: Thoroughbred

Founder: not stated

Phenotype: sabino-like to completely white

W23

Reference allele: C (N)

Mutant allele: G (W23)

Affected Breeds: Arabian

Founder: not stated (born 1989)

Phenotype: mostly white

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W30

Reference allele: T (N)

Mutant allele: A (W30)

Affected Breeds: Berber

Founder: Aghlasse

Phenotype: mostly white

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W31

Reference allele: No insertion (n)

Mutant allele: Insertion

Affected Breeds: Quarter Horse

Founder: Cookin Merada

Phenotype: White markings on face (bald face), legs, belly spots

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W32

Reference allele: C

Mutant allele: T

Affected Breeds: Quarter Horse

Discovered In: Small Town Scandal

Phenotype: White blaze/face marking, white socks, belly spots

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W34

Affected Breeds: Many

Founder: Unknown

Discovered In: Payback Flamboyant

Phenotype: White stockings, belly markings, and facial markings

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 Walleles 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.

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