Dominant White – W4

Description

White in horses is not a color in itself but an absence of pigment. Dominant white (W) is caused by variants in the KIT gene, which also underlies several other white spotting patterns. Dominant white is inherited as a dominant trait, so a horse needs only one copy of a W allele for it to be expressed. How much white a horse shows, and where, depends on which W allele it carries.

Dominant white is not a single variant but a family of different alleles of the KIT gene. Their effects range from an entirely white horse to a colored horse with white spotting. Dominant white is not albinism, which does not occur in horses: dominant white horses have dark eyes, and some keep colored hairs in the mane and tail.

Over 20 dominant white alleles have been described, named W1, W2, W3 and so on. Animal Genetics tests for the W3, W4, W5, W10, W13, W15, W19 and W20 alleles:

• W3 is found in Arabian horses and traces back to the near-white stallion R Khasper, born in 1996. A horse with one copy is usually all white or nearly white, with flecks of color in areas of the body. Two copies are considered possibly lethal, although this has not been confirmed in horses.
• W4 is found in the Camarillo White Horse and traces back to the stallion Sultan, a white horse born in 1912. Horses with W4 are white or nearly white, sometimes with pigmented areas along the topline that fade with age.
• W5 is found in Thoroughbreds and traces back to the stallion Puchilingui, born in 1984. It is highly variable and causes sabino-like spotting: some horses with one copy are all white, while others show a sabino-like pattern. Two copies are believed to be lethal.
• W10 is found in American Quarter Horses and traces back to the stallion GQ Santana, foaled in 2000. Like W5, it is highly variable: with one copy, some horses are all white and others show only modest spotting.
• W13 has been found in Quarter Horse and Peruvian Paso crosses, as well as other breeds, and produces a completely white horse. Two copies appear to be lethal.
• W15 is found in Arabian horses and is believed to have originated with the stallion Khartoon Klassic. Horses with one copy have sabino-like white markings, while horses with two copies are entirely white.
• W19 is found in Arabian horses. It causes bald face markings, white leg markings extending above the knees and hocks, and irregular belly spots. Horses with two copies are born mostly or entirely white.
• W20 was discovered in 2007 and is not specific to any one breed. On its own it causes modest white markings, but it increases the amount of white produced by other white alleles. For example, a horse with both W20 and W5 tends to show more white. Horses can carry one or two copies, as two copies are not lethal.

Testing can show whether a horse’s white coat or pattern is due to dominant white or to another white spotting gene, which is especially useful in horses that also carry patterns such as frame overo or tobiano. The test also shows whether a horse carries one or two copies of a W allele, which is important information for breeders.

If you believe your horse carries a dominant white allele not listed above, please call or email our office. We may be able to confirm whether the horse carries the specific KIT variant.

Possible results

Breeds Affected

Camarillo White Horse

Reference

Allelic heterogeneity at the equine KIT locus in dominant white (W) horses.

[PMID: 17997609]

Novel variants in the KIT and PAX3 genes in horses with white-spotted coat colour phenotypes.

[PMID: 23659293]

A novel splice mutation within equine KIT and the W15 allele in the homozygous state lead to all white coat color phenotypes.

[PMID: 28378922]

Related Tests

• Dominant White – W20 — the most widespread dominant white variant; adds white to markings
• Sabino1 — another white spotting pattern caused by a KIT variant
• Tobiano — a common white pattern also linked to the KIT gene