Joint distribution of the genotypes of KRT25 and SP6 variants demonstrating the epistatic effect of KRT25. The KRT25 variant NC_009154.2:g.21891160G>A is phenotypically fully expressed despite the presence of SP6 variant NC_009154.2:g.24022045C>T. A curly coated horse with complete hypotrichosis is shown to harbor a homozygous mutant KRT25 genotype (a). Curly horses with incomplete hypotrichosis show a mutant allele only in KRT25 variant or both in KRT25 and SP6 variant (b). Curly horses with no hypotrichosis have either a heterozygous or homozygous mutant genotype for SP6 variant (c). Straight coated horses have a homozygous wild type genotype in both loci (d). Grey coloring and crosses mark the genotypic distribution identified in this study and its correlation with the phenotypes.