Genetics and characteristics
Splashed White is a general name given to a number of variable white spotting patterns caused by mutations in two genes, the MITF gene and the PAX3 gene, in horses. Currently, there are six known mutations that cause a broad blaze, extended white markings on the legs, variable white spotting on the belly, and often blue eyes in different horse breeds. Mutations in genes responsible for the splashed white pattern have a pleiotropic effect meaning they may also cause some other traits or defects, horse deafness in this case. Splashed White 1 (SW1) is the first mutation reported to cause a variable white spotting pattern and is caused by a 10 bp insertion in the MITF promotor that regulates its expression and results in a disruption of the normal gene production by impacting pigment cell division.
This type of splashed white SW1 in horses is inherited as an autosomal dominant trait. A horse with only one copy of the mutated gene will have a variable white spotting pattern. Horses with two mutated genes, so-called double mutants, are very likely embryonic lethal. Early detection by genetic testing can help identify carriers of the mutation and help breeders in further selection of mating pairs.
Results Reported As
Test Result |
Interpretation of test result |
Normal coat color |
Horse does not have the gene with SW associated mutation and has a normal coat color. |
Splashed white pattern |
Horse has one copy of the gene with SW associated mutation and displays a splashed white pattern. |
Splashed white pattern or embryonic lethal |
Horse has two copies of the gene with the any of the SW associated mutations and displays a splashed white pattern. It is also possible that this combination is embryonic lethal. |
References:
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., 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. https://doi.org/10.1371/journal.pgen.1002653
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. The Journal of heredity, 111(3), 287–293. https://doi.org/10.1093/jhered/esaa009
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. https://doi.org/10.1111/age.12762