UC Davis School of Veterinary Medicine Veterinary Genetics Laboratory
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Dominant White Mutations – W5, W10 and W20

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The KIT gene has crucial function for the development of blood, gonadal and pigmentary tissues. Mutations that affect normal functioning of KIT gene products often result in lack of pigment cells (melanocytes) in the skin and hair follicles which leads to white patterning in horses known as Dominant White or White. Dominant White patterns are variable ranging from minimal Sabino-like spotting to all-white phenotypes. Eye color of Dominant White horses is typically brown. Studies of inherited white phenotypes in different breeds have shown that these arise as independent mutations. More than 20 different KIT mutations associated with white patterns have been identified to date. Except for W20, most of the known Dominant White mutations arose recently and are restricted to specific lines within breeds.

The Veterinary Genetics Laboratory offers a test for the W5, W10 and W20 mutations to owners who want to breed horses for Dominant White or to determine the genetic status of horses with unknown white patterns.

W5 is found in descendants of the Thoroughbred stallion Puchilingui. W10 is found in descendants of the Quarter Horse stallion GQ Santana. Because of the nature of the molecular change, it is thought that only horses that carry one copy of W5 or W10 are viable but this remains to be confirmed. W20 is a much older mutation that is found in many breeds and that has a subtle effect on the amount of white expressed. It appears to increase the expression of white in combination with other white pattern genes. Unlike W5 and W10, the homozygous condition (W20/W20) is not lethal.

Dominant White results are reported as:

Genotype

Interpretation

N/N

No evidence of altered sequences for W5, W10 or W20 mutations detected.

N/W5

One copy of W5 mutation detected. Horse will display some degree of white spotting but the specific pattern cannot be predicted.

W5/W10

One copy each of W5 and W10 detected. Horse will display white spotting and may be completely white.

W5/W20

One copy each of W5 and W20 detected. Horse will display white spotting and may be completely white.

W5/W5

Two copies of W5 mutation detected. Horse will display white spotting and may be completely white.*

N/W10

One copy of W10 mutation detected. Horse will display some degree of white spotting but the specific pattern cannot be predicted.

W10/W20

One copy each of W10 and W20 detected. Horse will display white spotting and may be completely white.

W10/W10

Two copies of W10 mutation detected. Horse will display white spotting and may be completely white.*

N/W20

One copy of W20 mutation detected. W20 has subtle or no effect on white spotting. Horse may display extended white markings.

W20/W20

Two copies of W20 mutation detected. W20 has subtle or no effect on white spotting. Horse may display extended white markings.

* Homozygous W5/W5 and W10/W10 horses may not be viable. This result may only be found in aborted fetuses produced in matings between two W5 or W10 carriers.

Reference

Haase B, Brooks SA, Schlumbaum A, Azor PJ, Bailey E, Alaeddine F, Mevissen M, Burger D, Poncet PA, Rieder S, Leeb T. Allelic heterogeneity at the equine KIT locus in dominant white (W) horses. PLoS Genet. Nov;3(11): e195, 2007.

Haase B, Brooks SA, Tozaki T, Burger D, Poncet P -A, Rieder S, Hasegawa T, Penedo C, Leeb T. Seven novel KIT mutations in horses with white coat colour phenotypes. Animal Genetics 40:623-629, 2009.

Haase B, Rieder S, Tozaki T, Hasegawa T, Penedo MCT, Jude R, Leeb T. Five novel KIT mutations in horses with white coat colour phenotypes. Animal Genetics 42:337-338, 2011.
 
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