The genetic correlation between two quantitative traits has been assumed to result from the sum of the effects of genes with ++, --, +-, -+, +0, -0,0+, and 0- effects on the two traits in question (see Lerner 1958). Rendel (1963) has suggested a somewhat different approach, namely that the correlation between traits could be considered in terms of the effects of genes on “make” (common resources available to the two traits) and on the distribution of these resources. Robertson and Reeve (1953) demonstrated that different chromosomes had different relative effects on the genetically correlated traits of wing and thorax length. However, there is little experimental information on the nature of the genetic correlations between traits. The object of this experiment was to determine the effects of various chromosomes on four genetically correlated bristle number traits. This was done by measuring the four bristle traits on stocks with combinations of chromosomes from two high abdominal bristle selection lines and their base population.