A series of directed crossings to test if the inheritance pattern of the trait matches multiple alleles. First visually different plants are crossed to create an F1; this generation is self-pollinated to create an F2 populations. In case of multiple alleles all F2 populations will segregate in a Mendelian 3:1 ratio, confirming that the three (or more) phenotypes relate to a monogenic trait.
This test may reveal epistasis (bigenic trait) instead of multiple alleles. Epistasis refers to two genes and therefore some populations (in F2, F3, etc) will segregate into three phenotypes.
RT: multiple alleles ↔ epistasis