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Much and perhaps most of the genetic information in the genomes of higher organisms is concerned with quantitative traits. Inherited quantitative differences, determined by many genes, are responsible in large measure for the distinction of individuals in a population and from an evolutionary point of view are probably more significant than qualitative differences determined by single genes. Due to the complexity of the genetic basis of quantitative traits, there is little experimental evidence concerning their genic background and control. A biochemical phenotype, such as enzymatic activity, is less complex than a phenotype at the morphological level, presumably is controlled by fewer genes, and has fewer steps between the primary genetic events and the observed phenotypes. It has previously been suggested' that dopa oxidase activity in Drosophila melanogaster may be considered a quantitative trait and that analysis of this system may provide information about the control and regulation of inherited quantitative differences at a molecular level. The evidence now presented shows that dopa oxidase activity in Drosophila is indeed a quantitative trait and that there may be two mechanisms operative: major controlling genes and regulating factors. These findings are discussed in relation to phenomena in microorganisms where the activity of a single enzyme is controlled by a number of interdependent genetic variables.2-4 Materials and Methods.-The dopa oxidase activity recorded in Tables 1 and 2 was measured in extracts of progeny from multiple matings; all other measurements of enzymatic activity were made on extracts prepared from the progeny of single-pair matings. Activity in homogenates of twenty flies or more can be reliably assayed. Flies up to 24 hrs. old were collected, frozen and stored under liquid nitrogen up to five days before the extracts were prepared according to the procedure described elsewhere.' In the survey of common laboratory strains, length of lag period and time of half. maximum activity were determined from activation curves obtained by assaying samples of extract during the activation process at 20-minute intervals. In all other experiments, extracts were incubated until fully activated before samples were removed for assay. All values of dopa oxidase activity are reproducible to within 10 per cent and the values cited are mean values of successive determinations from different homogenates of the designated strain of flies. Results.-Differences in activation rates and dopa oxidase activity among laboratory strains: Dopa oxidase is extracted from fly material in an inactive form complexed with another protein called "the activator." Following a lag period, the length of which is controlled in part by the concentration of initial free activator, activation proceeds by an autocatalytic process involving separation of activator and proen-

GENETIC CONTROL OF DOPA OXIDASE ACTIVITY IN DROSOPHILA MELANOGASTER, II. REGULATING MECHANISMS AND INTER-AND INTRA-STRAIN HETEROGENEITY