All white Shetland pony heterozygous for W13 (N/W13)
All white Shetland pony heterozygous for W13 (N/W13)

Test Update: new W13 allele added to VGL’s equine Dominant White test

The equine Dominant White test offered by the VGL has been updated to include a fifth allele: W13. This KIT mutation consists of a single base substitution and was initially identified in a Quarter Horse X Paso Peruano cross-bred family that had two white horses.

A study by the VGL's equine research team led by Dr. Rebecca Bellone, identified W13 in 14 all-white American Miniature Horses and an all-white Shetland Pony, indicating the mutation is present in multiple breeds. To date, no horses homozygous for W13 (W13/W13) have been identified, suggesting this may be embryonic lethal.

The KIT gene is crucial for the development of many cell types, including pigment cells (also called melanocytes), and mutations in this gene often result in decrease in melanocytes in the skin and hair follicles, leading to a white patterning in horses known as dominant white. It is believed that when a horse has two copies of some of these mutations (W5, W10, W13 or W22), there is a significant reduction or absence in KIT signaling that affects more than just the pigment cells and this is incompatible with life.

While to date 34 dominant white mutations have been identified in horses, most of them have been identified in a single family or breed.  The VGL has historically offered a test to identify the most common KIT mutations behind Dominant White phenotypes (W5, W10, W20, and W22).  However, given that W13 has been identified in additional horses and breeds, the test will now include W13.  While horses with the W5, W10, W20, and W22 mutations can have variable amounts of white patterning, horses with the W13 mutations are often reported to be all white with pink skin.

The VGL will add other dominant white variants to the test when additional research warrants it.  However, if you have a horse with a white pattern not explained by any of our tested mutations, please contact our research team. The VGL equine research team is always interested in exploring novel causes of white pattering!

Visit our test page for more information about Dominant White and the W13 allele or click here to review the latest study.