A pigment pathway with two switches

In a model fungus, colourless precursor is converted to yellow pigment by enzyme E1, then yellow to violet by enzyme E2. Functional alleles A and B encode E1 and E2 respectively; aa and bb have no activity. One functional copy suffices. All strains receive precursor, no pigment moves between colonies, and no alternative pathway exists. AaBb is crossed with AaBb; loci assort independently. A chemical rescue supplies yellow pigment inside the cells without affecting enzymes.