ABSTRACT
We have seen that genes which are related as hypo- and hypermorphs control the quantity of "effect" which occurs in the phenotype. In many cases, if not in all, these genes may be supposed to be responsible for the production of a greater or less quantity of some substance whose quantity controls the phenotypic effect. In most cases the nature of the substance in question is quite unknown; we have no idea, for instance, what substance may be involved in the control over bristle length shown by the gene bobbed. Indeed, this example raises the possibility that we may not be dealing with the quantity of a single chemical substance, but rather with the competence (the degree of reactivity to the bristle-forming stimulus) of a system consisting of many chemical parts.
