Created at the VGL Splashed White Mutations – SW1, SW2, SW3, SW4, SW5, and SW6

Quick Summary

Splashed white is a variable white spotting pattern characterized by a broad blaze, extended white markings on the legs, variable white spotting on the belly, and often blue eyes. Six mutations in two different genes have been shown to cause the splashed white pattern.

Foal with spalshed white markings
Foal with splashed white spotting pattern. This foal inherited the Splashed White 1 (SW1) variant from her dam. Image credit: courtesy of Ena Sparre

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Phenotype: Splashed white is a variable white spotting pattern characterized often by a large broad blaze, extended white markings on the legs, variable white spotting on the belly, and blue eyes.

Mode of Inheritance: Autosomal dominant

Alleles: N = Normal/Unaffected, SW1/SW2/SW3/SW4/SW5/SW6 = Splashed white 1/2/3/4/5/6

Breeds appropriate for testing: Appaloosa, Finnhorse, Icelandic Horse, Miniature Horse, Morgan Horse, Paint Horse, Quarter Horse, Shetland Pony, Thoroughbred, Trakehner, Pony of the Americas

Explanation of Results:

  • Horses with N/N genotype do not have the splashed white pattern caused by SW1, SW2, SW3, SW4, SW5, or SW6. If this horse has a white spotting pattern, it is caused by some other genetic mutation.
  • Horses with N/SW1, N/SW2, N/SW3, N/SW4, N/SW5, or N/SW6 genotype will have splashed white patterning. They may transmit a splashed white variant to 50% of their offspring. Matings with N/N genotype will result in a 50% chance of producing a foal with splashed white patterning.
  • Horses with two splashed white variants, which can be SW1/SW1, SW2/SW2, SW1/SW3, SW1/SW5*, SW3/SW5*, SW1/SW6, SW3/SW6*, SW5/SW6*, SW2/SW4, will have splashed white patterning and will transmit a splashed white variant to all of their offspring. Matings with any genotype are predicted to produce all splashed white offspring.

*Horses homozygous for the SW3, SW4, SW5, and SW6 variants (SW3/SW3, SW4/SW4, SW5/SW5, SW6/SW6 genotypes) have not been detected and may be embryonic lethal. In addition, the VGL has not identified any SW1/SW5, SW3/SW5, SW3/SW6, or SW5/SW6 compound heterozygotes. It is unknown at this time if these combinations are non-viable or just not yet detected because of the low frequency of SW5 and SW6.


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Additional Details

Splashed white is a variable white spotting pattern characterized primarily by a large, broad blaze, extensive white markings on legs, variable white spotting on belly, and often blue eyes. Some, but not all, splashed white horses are also deaf. Six mutations have been identified and have been named in the order they were discovered: SW1, SW2, SW3, SW4, SW5, and SW6. All of these mutations cause a similar splashed white phenotype in horses although the amount of white patterning is variable and thought to be controlled by other genes. Some are known, but mostly these additional genes, contributing to the variation in splash white phenotype, are yet to be identified.

MITF - SW1, SW3, SW5, and SW6

SW1, SW3, SW5, SW6 are all variants in the MITF (microphthalmia associated transcription factor) gene. MITF is an important protein for normal pigment cell function. It is a transcription factor that activates other genes in the pigment process including those known to regulate melanocyte proliferation and migration.

SW1 is found in several breeds – Quarter Horse, Appaloosa, Paint Horse, Morgan Horse, Trakehner, Miniature Horse, Shetland Pony and Icelandic Horse – and may be present in other breeds as well. The SW1 variant is a 10 bp insertion in the MITF promotor (chr16:g.20,117,302Tdelins11). The promoter is the region of the gene that regulates gene expression. SW1 is thought to disrupt the normal gene production thus impacting pigment cell division. Horses homozygous for SW1 (SW1/SW1) have been identified, which suggests that this mutation is not homozygous lethal.

SW3 occurs exclusively in certain lines of Quarter Horses and Paint Horses and is caused by a frameshift mutation MITF (p.C280Sfs*20). SW3 is thought to be lethal in the homozygous state (SW3/SW3) as no individuals with this genotype have yet been identified. The current recommendation is that mating of two horses that carry SW3 should be avoided.

SW5 is a large deletion of 63 thousand nucleotides in the MITF gene (chr16:21,503,211–21,566,617). This variant removes several important regions of the gene and likely causes the gene to have a reduced or complete loss of function. This mutation was identified in an American Paint Horse family. Subsequent research at the VGL, suggest the frequency of this mutation is low in the Paint Horse breed. It is unknown if SW5 is homozygous lethal and no homozygotes have been reported.

SW6 was recently identified by research at the Veterinary Genetics Laboratory and like SW5 is a large deletion of the MITF gene, that likely inhibits proper function of the gene product. This deletion removes 8710 nucleotides of the gene. It is predicted that this change in the DNA results in a shortened MITF protein (the product of the MITF gene). The region that is removed normally binds to the DNA of other pigmentation genes to activate them. The end result is white areas lacking pigment cells. At this time, it is not known whether homozygosity for SW6 (two copies of this mutation) is viable. Based on the type of mutation and the role MITF plays in development, we suspect that SW6/SW6 may be embryonic lethal. However one SW1/SW6 compound heterozygote was identified as part of the VGL study.

SW6 was identified in a single family. An exciting finding to our study was identifying SW6 as a de novo (new/recent) mutation. More specifically, a de novo mutation means that the change in the DNA occurred either during the cellular processes that gave rise to the egg or sperm or during early embryonic development of the stallion investigated in the study. Being a de novo mutation also means only this individual, his offspring, and resulting future generations of horses originating with this stallion can have this mutation. Identification of a de novo mutation was an exciting finding because we know precisely the individual and generation in which the mutation occurred; this is not something we often get to determine.

PAX3 - SW2 and SW4

SW2 and SW4 are mutations in the PAX3 (paired box gene 3) gene. SW2 is found in certain lines of Quarter Horses and Paint Horses and is caused by a missense mutation in PAX3 (p.Cys70Tyr). PAX3 is essential for proper development of some nerve tissue and pigment cells. Based on testing results from the VGL, SW2 is currently considered not homozygous lethal, though horses that have SW2 may be deaf. The rare SW4 mutation has been identified in family of Appaloosa horses and may cause a splashed white or a broad blaze. SW4 is caused by a missense mutation in PAX3 (p.Pro32Arg), and is also thought to be lethal in the homozygous state, though this remains to be determined.

Splashed white mutations are inherited as dominant traits with variable expression, which means that one copy of a SW mutation will produce a white spotting phenotype with variable amount of white. Horses that carry combinations of the splashed white mutations, tobiano, or lethal white overo can display extensive white patterning or their coats maybe entirely white.

Testing for the splashed white pattern allows breeders to identify which splashed white mutation is present in their horse and can aid in breeding decisions and clinical decisions if deafness is suspected.

Type of Sample


Type of Test

Results Reported As

Test Result


Splashed White (SW1, SW3, SW5, SW6)


No copies of SW1, SW3, SW5, or SW6 Splashed White mutations detected.


One copy of SW1 detected.


Two copies of SW1 detected.


One copy of SW3 detected.


Two copies of SW3 detected.*


One copy of SW5 detected.


Two copies of SW5 detected.*


One copy of SW6 detected.


Two copies of SW6 detected.*


One copy of SW1 and one copy of SW3 detected.


One copy of SW1 and one copy of SW5 detected.#


One copy of SW1 and one copy of SW5 detected.


One copy of SW3 and one copy of SW5 detected.#


One copy of SW3 and one copy of SW6 detected.#


One copy of SW5 and one copy of SW6 detected.#

Splashed White (SW2, SW4)


No copies of SW2 or SW4 detected.


One copy of SW2 detected


Two copies of SW2 detected.


One copy of SW4 detected.


Two copies of SW4 detected.*


One copy of SW2 and one copy of SW4 detected.

*Homozygous genotypes for the SW3, SW4, SW5, and SW6 mutations (SW3/SW3, SW4/SW4, SW5/SW5, SW6/SW6) may be embryonic lethal.

# These combinations of splashed white have not been observed, either due to low frequency of the SW5 and SW6 alleles or embryonic lethality.


Magdesian, K.G., Williams, D.C., Aleman, M., Lecouteur, R.A., & Madigan, J.E. (2009). Evaluation of deafness in American Paint Horses by phenotype, brainstem auditory-evoked responses, and endothelin receptor B genotype. Journal of the American Veterinary Medical Association235(10), 1204-1211. doi: 10.2460/javma.235.10.1204

Hauswirth, R., Haase, B., Blatter, M., Brooks, S. A., Burger, D., Drögemüller, C., Gerber, V., Henke, D., Janda, J., Jude, R., Magdesian, K. G., Matthews, J. M., Poncet, P. A., Svansson, V., Tozaki, T., Wilkinson-White, L., Penedo, M. C. T., Rieder, S., & Leeb, T. (2012). Mutations in MITF and PAX3 cause ‘‘Splashed White’’ and other white spotting phenotypes in horses. Plos Genetics, 8(4): e1002653. doi: 10.1371/journal.pgen.1002653

Hauswirth, R., Jude, R., Haase, B., Bellone, R. R., Archer, S., Holl, H., Brooks, S. A., Tozaki, T., Penedo, M. C., Rieder, S., & Leeb, T. (2013). Novel variants in the KIT and PAX3 genes in horses with white-spotted coat colour phenotypes. Animal Genetics, 44(6), 763-765. doi: 10.1111/age.12057

Henkel, J., Lafayette, C., Brooks, S.A., Martin, K., Patterson-Rosa, L., Cook, D., Jagannathan, V., & Leeb, T. (2019). Whole‐genome sequencing reveals a large deletion in the MITF gene in horses with white spotted coat colour and increased risk of deafness. Animal Genetics, 50(2), 172-174. doi: 10.1111/age.12762

Magdesian, K.G., Tanaka, J., & Bellone, R.R. (2020). A De Novo MITF Deletion Explains a Novel Splashed White Phenotype in an American Paint Horse. Journal of Heredity, 111(3), 287-293. doi: 10.1093/jhered/esaa009

Kvist, L., Honka, J., Niskanen, M., Liedes, O., & Aspi, J. (2020). Selection in the Finnhorse, a native all‐around horse breed. Journal of Animal Breeding and Genetics, 00: 1-16. doi: 10.1111/jbg.12524