ABSTRACT
A human and an elephant have essentially the same reactants available to them – a mixture of gases, minerals, and biological compounds. If every reaction were identical and proceeded equally fast in both the human and elephant, the two animals would have the same chemistry and would look alike. The essential distinction between a dark-haired person and a fair-haired person is not in the food they eat or the air they breathe – those are shared in common; the difference is in the presence of enzymes that catalyze reactions leading to the formation of certain pigment molecules found in hair. The discussion on the enzymatic determination of organismal chemistry is a conceptually simple one, resting entirely on chemical effects, and in a more realistic description we would have to include some other factors. For instance, two chemically identical organisms will probably be phenotypically different if they are placed in different environments; the expressions of many genes are sensitive to environmental factors.
