Abstract
Chestnut coat color in horses is determined by a missense mutation within the MC1R gene. However, the intensity of the chestnut color can vary widely within individuals possessing this genotype. Here, we investigated this variation using standardized photographs of 96 horses. Each horse was ranked lightest to darkest within the cohort for phenotype by 3 blinded observers. A genome-wide association study utilizing the relative shade ranking as the phenotype and using 268 487 single-nucleotide polymorphisms (SNPs) genotyped using the Affymetrix Equine 670k array identified a single significantly associated region on chromosome 3 (P = 2.934 × 10-8). Analysis of whole-genome sequences for horses spanning the diverse range of chestnut color identified candidate SNPs within the coding sequence of the only gene in the region: SALL1. The function of SALL1 is largely unknown, though it is predicted to interact with the Hermansky-Pudlak Syndrome type 1 (HPS1) protein, which causes partial albinism in humans. However, with only one study suggesting a circumstantial influence of the SALL1 protein on pigmentation, additional work is needed to confirm this new coat color locus in larger populations and investigate the function of this protein for impacts on equine health.