W H I T E S P O T T I N G
White spotting was previously known as dominant white, renamed because not all of the mutations cause a mostly-white or all-white phenotype. They are various types of missense, nonsense, and SNP mutations on KIT. As such they could be considered the differently named sabino mutations 2-23. KIT is an extremely unstable gene and seems to mutate easily and relatively often. There are currently twenty two mapped mutations of white spotting. Some white spotting mutations, such as in Camarillo White Horses, are homozygous lethal. Others are not. Most white spotting mutations are breed-specific, and more so family-specific.
W#W# or nW#
I was unable to include examples of some mutations due to lack of clear photographs. These would be W8 (Thokkadis Icelandics), W9 (unknown Holsteiner), W13 (unidentified Quarter Horses), and W16 (unidentified Oldenburgs). The majority of mutations, however, I have been able to include in the gallery.
W1 Cigale line of Freibergers; nonsense mutation on Exon15.
W2 KY Colonel line of Thoroughbreds; missense mutation on Exon17.
W3 R Khasper line of Arabians; nonsense mutation on Exon4.
W4 Sultan line of Spanish Mustangs; missense mutation on Exon12.
W5 Puchilingui line of Thoroughbreds; frameshift mutation on Exon15.
W6 Marumatsu Live line of Thoroughbreds; missense mutation on Exon5.
W7 Turf Club line of Thoroughbreds; splice site mutation on Intron2.
W10 GQ Santana line of Quarter Horses; frameshift mutation on Exon7.
W11 Schimmel line of South German Drafts; splice site mutation on Intron20.
W12 Painting the Sky line of Thoroughbreds; deletion mutation on Exon3.
W14 Shirayukihime line of Thoroughbreds; deletion mutation on Exon17.
W15 Rhocky Rhoad line of Arabians; missense mutation on Exon10.
W17 Hakubahime line of Japanese Drafts; missense mutation on Exon14.
W18 Colorina von Hof line of Swiss Warmbloods; splice site mutation on Intron8.
W19 Fantasia Vu line of Arabians; missense mutation on Exon8.
W20 Various breeds; missense mutation on Exon14
W21 Undisclosed line of Icelandic Horses; frameshift mutation on Exon17.
W22 Airdrie Apache line of Thoroughbreds; no published details yet.
W1 Cigale line of Freibergers; nonsense mutation on Exon15.
W2 KY Colonel line of Thoroughbreds; missense mutation on Exon17.
W3 R Khasper line of Arabians; nonsense mutation on Exon4.
W4 Sultan line of Spanish Mustangs; missense mutation on Exon12.
W5 Puchilingui line of Thoroughbreds; frameshift mutation on Exon15.
W6 Marumatsu Live line of Thoroughbreds; missense mutation on Exon5.
W7 Turf Club line of Thoroughbreds; splice site mutation on Intron2.
W10 GQ Santana line of Quarter Horses; frameshift mutation on Exon7.
W11 Schimmel line of South German Drafts; splice site mutation on Intron20.
W12 Painting the Sky line of Thoroughbreds; deletion mutation on Exon3.
W14 Shirayukihime line of Thoroughbreds; deletion mutation on Exon17.
W15 Rhocky Rhoad line of Arabians; missense mutation on Exon10.
W17 Hakubahime line of Japanese Drafts; missense mutation on Exon14.
W18 Colorina von Hof line of Swiss Warmbloods; splice site mutation on Intron8.
W19 Fantasia Vu line of Arabians; missense mutation on Exon8.
W20 Various breeds; missense mutation on Exon14
W21 Undisclosed line of Icelandic Horses; frameshift mutation on Exon17.
W22 Airdrie Apache line of Thoroughbreds; no published details yet.
Among the white spotting mutations, W20 in particular is interesting in its function. It appears to be incomplete dominant, with very minimal markings on heterozygous animals and more chrome and belly spots on homozygous animals. It doesn't appear to express much on its own. It has been determined that W20 acts as a white booster for other patterns. It has been found that horses with both W20 and another W-numbered mutation tend to have louder white expression than others that are heterozygous for their W-numbered mutation and don't have W20.