W20 is a white booster. It will increase the amount of white on a horse that carries another white spotting pattern.
Dominant White 20
Breeds known to carry this mutation
W20 by itself only causes small white markings such as socks and a blaze. However, when in combination with another dominant white spotting marker (notified by ‘W’ followed by a number), it will increase the amount of white on that horse. It can cause a horse that would be partially white (from W5 or W22 for example) to become completely white. It appears to be an old mutation because it is found in many breeds.
Interpretation of results
n/n: Horse does not carry the W20 mutation.
W20/n: Horse carries one copy of the W20 mutation and likely has white socks and facial markings. If the horse also carries another ‘W’ marker, it might be completely white.
W20/W20: Horse carries two copies of the W20 mutation. It likely has white socks and facial markings. It will pass the W20 mutation on to all of its offspring.
Hauswirth et al. Novel variants in the KIT and PAX3 genes in horses with white‐spotted coat colour phenotypes. Animal Genetics 2013; 44:763-765. doi: 10.1111/age.12057