The foregoing series of experiments, demonstrating the way in which the phenotypedark arises by inheritance, has shown that this type depends upon the presence of at least two genes: (1) the well-known recessive non-agouti, a, in the heterozygous condition; and (2) one or more dominant or semi-dominant modifying factors, U1 [U2…], whose effect is to make non-agouti partially dominant to agouti.