UC Davis School of Veterinary Medicine Veterinary Genetics Laboratory
Dominant White Mutations – W5, W10, W20, and W22

Order this test on MyVGL

Quick Summary

Dominant white is a variable white spotting pattern caused by many different mutations in the KIT gene. The VGL tests for the four most common mutations known as W5, W10, W20, and W22. Homozygosity for W5, W10, or W22 is thought to be non-viable.

Phenotype: Dominant white causes a variable white coat patterning phenotype that typically ranges from extensive white face and leg markings with or without roaning and/or white patches in the belly to a nearly all white horse.

Mode of Inheritance: Autosomal dominant

Alleles: N = Normal, W5 = Dominant white 5, W10 = Dominant white 10, W20 = Dominant white 20 and W22 = Dominant white 22.

Breeds appropriate for testing: W5, W22 = Thoroughbred and Thoroughbred crosses. W10 = Quarter Horses and related breeds including Paint Horses, Appaloosas, and Pony of the Americas. W20 has been identified in many horse breeds so nearly all breeds are appropriate for testing.

Explanation of Results:

  • Horses with N/N genotype do not have a white spotting pattern caused by W5, W10, W20, or W22. If this horse has a white spotting pattern, it is caused by some other genetic mechanism.
  • Horses with N/W5, N/W10, or N/W22 genotypes will display a dominant white spotting pattern that can vary from large amounts of white face and leg markings to a horse that is almost completely white.
  • Horses with W5/W20, W10/W20, or W22/W20 genotypes will display a dominant white spotting pattern that are typically all white.
  • Horses with N/W20 or W20/W20 genotype display white face and leg markings and some may have a variable amount of white spotting. It is thought that horses with these genotypes that have more extreme white spotting patterns, likely have mutations in other pigmentation genes.
  • Horses with W5/W5, W10/W10, W22/W22 are thought to be embryonic lethal, therefore it is advisable to not mate horses with W5, W10, or W22 alleles to each other.

Additional Details

The KIT gene has crucial function for the development of many cell types, including blood and pigment cells (melanocytes). Mutations that affect normal functioning of the KIT protein often result in lack of melanocytes in the skin and hair follicles, which leads to white patterning in horses known as dominant white.

Dominant white patterns are variable ranging from extensive face and leg markings with or without minimal sabino like patterns, including roaning on the belly and/or belly spots, to an all-white horse. Eye color of dominant white horses is typically brown.

A number of different KIT mutations associated with white patterns have been identified in the horse. These include dominant white, Sabino-1, and Tobiano. To date, 28 of these mutations have been characterized as dominant white mutations. Dominant white mutations, with the exception of W20, arose recently and are restricted to specific lines within breeds.

W5 (c.2193delG) is found in descendants of the Thoroughbred stallion Puchilingui. W10 (c.1126_1129delGAAC) is found in descendants of the Quarter Horse stallion GQ Santana. W22 (Chr3:79548925-79550822del1898insTATAT) is found in descendants of the Thoroughbred stallion Airdrie Apache.

The majority of the dominant white mutations identified are thought to be detrimental to KIT protein function. When horses have one copy of these mutations they have a reduced number of melanocytes and thus have a white spotting pattern. However, when they have two copies of the mutation (homozygous), it is believed that reduction or absence of KIT signaling occurs, affecting more than just the pigment cells. For example, horses homozygous W5, W10, and W22 are likely embryonic lethal. However, this remains to be confirmed. It is unknown if horses that are compound heterozygous (two different mutations in the same gene, i.e. W5/W10, W5/22, or W10/W22) are viable, and testing through the VGL has yet to identify horses with these genotypes.

W20 (c.2045G>A) is a much older mutation and is found in many breeds. This mutation is thought to have a more minor effect on protein function as well as a subtler effect on the amount of white expressed unless in combination with other dominant white alleles. In combination with other white pattern alleles, W20 has been shown to increase the amount of white patterning, producing an all white or nearly all white phenotype. Unlike W5, W10 and W22, the homozygous condition W20/W20 is not lethal.

W22 occurs on the W20 background, that means that all horses with the W22 mutation also have the W20 mutation. Since the W22 mutation has a greater impact on protein function than W20, the reported allele is W22. It is possible for horses to be compound heterozygotes, W20/W22, and these have been reported to have an all-white phenotype.

The Veterinary Genetics Laboratory offers tests for W5, W10, W20, and W22 mutations to owners who want to breed horses for dominant white or to determine the genetic status of horses with unknown white patterns.

Dominant white results reported as:

Genotype#

Interpretation

N/N

No evidence of W5, W10, W20, or W22 mutations detected.

N/W5

One copy of W5 mutation detected. Horse will display some degree of white spotting but the specific pattern cannot be predicted.

W5/W20

One copy each of W5 and W20 detected. Horse will display white spotting and may be completely white.

N/W10

One copy of W10 mutation detected. Horse will display some degree of white spotting but the specific pattern cannot be predicted.

W10/W20

One copy each of W10 and W20 detected. Horse will display white spotting and may be completely white.

N/W20

One copy of W20 mutation detected. W20 has subtle or no effect on white spotting. Horse may display extended white markings.

W20/W20

Two copies of W20 mutation detected. W20 has subtle or no effect on white spotting. Horse may display extended white markings.

W20/W22

One copy each of W20 and W22 detected. Horse is completely white or nearly so.

N/W22

One copy of W22 mutation detected. Horse will display some degree of white spotting but the specific pattern cannot be predicted.

W5/W5
W10/W10
W22/W22

Two copies of W5, W10, or W22 detected. Horse will display white spotting and may be completely white.*

W5/W10
W5/W22
W10/W22

Two different copies of the W5, W10, or W22 mutations. Horse will display white spotting and may be completely white.#

* Homozygous W5/W5, W10/W10, or W22/W22 horses may be embryonic lethal and thus not viable. This result may only be found in aborted fetuses produced in matings between two horses that have W5, W10 or W22 variants. Similarly, compound heterozygotes for these variants may also be embryonic lethal.
# Not all expected combinations of dominant white mutations have been observed but are possible if mating pairs possess different variants.

References

Dürig N, Jude R, Holl H, Brooks SA, Lafayette C, Jagannathan V, Leeb T. (2017) Whole genome sequencing reveals a novel deletion variant in the KIT gene in horses with white spotted coat colour phenotypes. Animal Genetics 48(4):483-485. doi: 10.1111/age.12556

Haase B, Brooks SA, Schlumbaum A, Azor PJ, Bailey E, Alaeddine F, Mevissen M, Burger D, Poncet PA, Rieder S, Leeb T. (2007) Allelic heterogeneity at the equine KIT locus in dominant white (W) horses. PLoS Genetics Nov;3(11): e195. doi: 10.1371/journal.pgen.0030195

Haase B, Brooks SA, Tozaki T, Burger D, Poncet P -A, Rieder S, Hasegawa T, Penedo C, Leeb T. (2009) Seven novel KIT mutations in horses with white coat colour phenotypes. Animal Genetics 40:623-629. doi: 10.1111/j.1365-2052.2009.01893.x

Haase B, Rieder S, Tozaki T, Hasegawa T, Penedo MCT, Jude R, Leeb T. (2011) Five novel KIT mutations in horses with white coat colour phenotypes. Animal Genetics 42:337-338. doi: 10.1111/j.1365-2052.2011.02173.x

Hauswirth, R., Jude, R., Haase, B., Bellone, R.R., Archer, S., Holl, H., Brooks, S.A., Tozaki, T., Penedo, M.C.T., Rieder, S. and Leeb, T. (2013), Novel variants in the KIT and PAX3 genes in horses with white-spotted coat colour phenotypes. Anim Genet, 44: 763-765. doi:10.1111/age.12057

Negro, S., Imsland, F., Valera, M., Molina, A., Solé, M. and Andersson, L. (2017), Association analysis of KIT, MITF, and PAX3 variants with white markings in Spanish horses. Anim Genet, 48: 349-352. doi:10.1111/age.12528

Veterinary Genetics Laboratory, Tel 530-752-2211, Email VGL