Swallowtail butterfly tiger swallowtail), Y-linkage determines whether the females are either wild-type (yellow and black) or melanic (dark melanin replaces the yellow background). This genetic difference stems from the fact that melanism is controlled by a single gene, which controls the level of dopamine in the organism. The enzyme BAS, which assists dopamine in producing the yellow pigmentation, normally found on the wings' background, is suppressed. Without the pigmentation, the butterfly appears mostly black (the melanic form) and is a Batesian mimic of "Battus philenor", the pipevine swallowtail. There are also "Papilio glaucus" that are not wholly black; several possess an intermediate
Comma category difference between the two notions is that a natural transformation is a particular collection of morphisms of type of the form formula_182, while objects of the comma category contains "all" morphisms of type of such form. A functor to the comma category selects that particular collection of morphisms. This is described succinctly by an observation by Huq that a natural transformation formula_183, with formula_184, corresponds to a functor formula_185 which maps each object formula_8 to formula_187 and maps each morphism formula_188 to formula_16. This is a bijective correspondence between natural transformations formula_181 and functors formula_185 which are sections of both