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 functors out of the product category formula_97. For each comma category there are forgetful functors from it. Several interesting categories have a natural definition in terms of comma categories. Limits and colimits in comma categories may be "inherited". If formula_1 and formula_2 are complete, formula_145 is a continuous, and formula_146 is another functor (not necessarily continuous), then the comma category formula_6 produced is complete , and the projection functors formula_148 and formula_149 are continuous. Similarly, if formula_1 and formula_2 are cocomplete, and formula_152 is cocontinuous, then formula_6 is cocomplete, and the projection functors are cocontinuous. For example, note that in