Abstract
The American Paint Horse Association (APHA) officially records pedigree and performance information for their breed; these registered stock-type horses are valued for utility in work on the farm and ranch and as pleasure horses. As the name of the breed implies, the breed is also valued for attractive white spotting patterns on the coat. Current APHA procedures utilize visual inspections of photographs to determine if white spotting phenotypes exceed threshold anatomical landmarks considered by the association as characteristic of desirable patterns. Horses with sufficient white patterning enter the “Regular” registry, rather than the “Solid Paint-Bred” division. Genetic studies have now defined causative sequence variants for 35 alleles likely contributing to white spotting phenotypes in the horse. Here, the allele frequency was calculated for nine white spotting alleles commonly found in the American Paint horse breed using a sample of 1,065 APHA registered animals. Epistatic interactions were documented among five spotting loci, and with the MC1R and ASIP genes controlling black and red pigment production. The W20 allele within the KIT gene, in the absence of other spotting loci, was strongly associated with the APHA-defined white spotting phenotype (p = 1.86 x10−18), refuting advertisements reporting that W20 acts only as a modifier of other white spotting patterns. An empirical definition of the action of these loci on the desired APHA-defined white spotting phenotype will allow for a more accurate application of genome-assisted selection in this breed, providing breeders with a powerful tool to increase the marketability of their horses.
