Abstract
The nature of gene action involved in the mode of determination of two quantitative traits of Drosophila melanogaster, e.g., number of ovarioles and sternite chaetae, was studied in freshly captured strains and their crosses, using the biomodel built up by Cockerman and Weir. The results proved to be similar to those obtained with laboratory populations. The genome is by far the most important source of variation for the two traits. Extragenetic interactions are far more important in freshly captured stock than in flies which have already been propagated for several years in the laboratory. We can now interpret neutrality of a quantitative character as a state of equilibrium between medium and population. Since there are extragenetic interactions (type k) in the case of the neutral character, we could even think of possible future variations able to alter its mean value in such a way as to make it selective.