Although typically assumed to be a color, white is actually coded in DNA as a deletion of color. In horses, the dominant white gene (DW) is controlled by a single dominant locus, the W locus. The W locus occurs in the KIT gene, which controls several other white pattern traits. The DW gene is a dominant trait. This means that a horse only has to inherit one copy of the gene in order for the trait to be expressed. The DW gene deletes the color entirely from the horse. However, the amount deleted and the location the color is deleted from depends on the specific allele the horse inherits.
The DW gene is technically a group of genes that have different effects on different species. These effects range from producing entirely white horses to producing colored horses with white patterns. DW coloration is not to be confused with albinism, which does not occur in horses. DW leaves the eyes dark and different colored manes and tails have been recorded.
There are 20 common alleles that are grouped under the dominant white gene, known as W1-W20. At Animal Genetics, we test for the W3, W5, W10, W13, W15, and W20 alleles. The descriptions of these alleles are found below:
W3 – The W3 allele is typically found in Arabian horses. A horse with one copy of the W3 allele is typically all white with interspersed flecks throughout regions of their body. However, a horse that inherits two copies of this allele will not survive.
W5 – The W5 allele can be found in Thoroughbreds. This is a highly variable allele, and causes sabino-like spotting on the horse. There are some horses with one copy of this allele that have been all white and some that have had a pattern similar to sabino. If a horse has two copies of the W5 allele, the horse will not survive.
W10 – The W10 allele is typically found in American Quarter Horses. Similar to the W5 allele, this allele is also highly variable. With one copy of this allele, some horses are all white and some only exhibit modest spotting. If a horse has two copies of this allele, the horse will not survive.
W13 – The W13 allele is found in Quarter horses. This allele causes a completely white horse. A horse can have either one or two copies of this allele, as having two copies is not lethal.
W15 – The W15 allele is found in Arabian horses and is believed to have originated with the Stallion Khartoon Klassic. Horses heterozygous for W15 will have sabino-like white markings while homozygotes appear entirely white.
W19 – The W19 allele is found in Arabian horses. This allele causes bald face markings, white leg markings extending above the knees and hocks, and irregular belly spots.
W20 – The W20 allele was discovered in 2007. This allele is not specific to any one breed of horse, but rather influences the degree of white pattern other alleles can have on horse patterns. For example, the W20 allele is found to influence the degree of white in a horse that carries the W5 allele, causing the horse to develop more of the white pattern. A horse can have one or two copies of this allele, as having two copies is not lethal.
Testing can determine if the coat pattern of a horse is actually dominant white (W3, W5, W10, W13, W15, and W20) or if a different modifier is present. This can be particularly useful in horses that have additional patterns, such as frame overo and tobiano. DNA testing for dominant white (W3, W5, W10, W13, W15, and W20) also identifies if a horse has 1 or 2 copies of the gene, which can be an informative tool for breeders.
If you believe your horse is contains the genetic makeup for any of the dominant white alleles (W1-W20), with special regards to the alleles listed above, please feel free to reach out to us. Animal Genetics is able to confirm if the horse carries the specific KIT mutation related to these alleles. Please call our office or email us for more information.
Allelic heterogeneity at the equine KIT locus in dominant white (W) horses. Haase B, Brooks SA, Schlumbaum A, Azor PJ, Bailey E, Alaeddine F, Mevissen M, Burger D, Poncet PA, Rieder S, Leeb T. PLoS Genet. 2007 Nov;3(11):e195. [PMID: 17997609]
Novel variants in the KIT and PAX3 genes in horses with white-spotted coat colour phenotypes. Hauswirth R, Jude R, Haase B, Bellone RR, Archer S, Holl H, Brooks SA, Tozaki T, Penedo MC, Rieder S, Leeb T. Anim Genet. 2013 Dec;44(6):763-5. [PMID: 23659293]
A novel splice mutation within equine KIT and the W15 allele in the homozygous state lead to all white coat color phenotypes. Holl H, Brooks S, Carpenter M, Bustamante C, Lafayette C. Anim Genet. 2017 Feb;48(4)497-498. [PMID: 28378922]
|W4/W4||Horse carries two copies of the Dominant White 4 mutation.|
|W4/n||Horse carries one copy of the W4 Dominant White mutation.|
|n/n||Horse is negative for the W4 Dominant White mutation.|