One pigment locus, designatedP, governs the occurrence of an acylated glycoside of petunidin in tuber, sprout and flower, and in this respect corresponds to the ‘wild-type’ locus of wild tuberous species in which the same derivative of petunidin occurs alone.

Two alleles of the other pigment locus, designatedR, are identified and are hypostatic toP. The dominant,R, in the absence of P, has a dual effect: an acylated glycoside of pelargonidin occurs in the tuber and sprout only, and a glycoside of cyanidin in the flower only. Epistasy is incomplete in the flower where the glycoside of cyanidin occurs also in the presence ofP. These multiple effects are inseparable in heredity. In individuals homozygous for the recessiveP^pw, and in the absence ofP, these pigments do not occur.

An acylated derivative of peonidin occurs in all cultivated diploids, and in consequence no conclusions can be drawn about its inheritance. This anthocyanin is suppressed in the flowers only of genotypes homozygous forR^pw. The suggestion is made that further search might reveal an alleler which in the homozygous phase and in the absence ofP would be expected to suppress all pigments, at least in tubers and flowers.

Types of anthocyamdins other than petunidin occur only in cultivated potatoes but are similar in diploids and tetraploids. Presumably, this variability has been established by human selection at the diploid level and the evolution of tetraploidy was a later event in the genetic history of cultivated potatoes.