W22 is one of the many mutations of the KIT gene that cause the Dominant White phenotype.
Dominant White 22, White spotting W22
Breeds known to carry this mutation
Thoroughbreds. It is thought that W22 originated in the mare Not Quite White (1989), but it has been widely distributed through her son Airdrie Apache.
W22 causes its carriers to have a significant amount of white which can range from a large blaze and white legs, to a sabino-like pattern, to the horse being completely white.
W22 occurred on a copy of the KIT gene that already carried the W20 mutation, so horses with W22 also carry W20 (so they have both W20 & W22, although this is just reported as W22).
If a horse carries W22/W20 on one chromosome, and also inherits W20 from their other parent, they are often completely white.
Interpretation of results
n/n: Horse does not carry the W22 mutation.
W22/n: Horse carries one copy of the W22 mutation and will have a significant amount of white in its coat. It will pass the W22 mutation on to approximately half of its offspring.
W22/W22: Horse carries two copies of the W22 mutation. Please note that this is a theoretical genotype, and no horse has been identified as carrying two W22 mutations. It is suspected that embryos carrying two W22 mutations are not viable and die early in the pregnancy.
Durig et al. Whole genome sequencing reveals a novel deletion variant in the KIT gene in horses with white spotted coat colour phenotypes. Animal Genetics 2017; 48:483-485. doi: 10.1111/age.12556.