Epistasis (Greek: 'standing upon') occurs when one gene (the epistatic gene) masks or suppresses the phenotypic expression of another gene at a different (non-allelic) locus (the hypostatic gene). This is a form of gene-gene interaction (non-allelic interaction). Classic examples: coat colour genes in Labrador dogs; feather colour in chickens. Epistasis modifies expected Mendelian ratios (9:3:3:1) to produce ratios like 9:7, 12:3:1, 9:3:4, or 15:1.