The effects of chromosome substitution on male body weight of Drosophila melanogaster.

HE manifestation of a quantitative trait is dependent upon the relationship Tof a number of genes in the same and different chromosome pairs. The nature of these relationships can be studied by comparing the effects of homologous chromosomes from different strains in a controlled genetic background. In experiments of this kind, the effects of the genes involved can be described in terms of effects of single chromosomes and of interaction between different chromosomes. A simplifying assumption which has been made frequently in the past is that there are a number of genes involved in the realization of the characters which act in a plus or minus direction. Depending on the balance of plus and minus genes in a particular chromosome, the chromosome itself will have a plus or minus effect. The plus and minus effects of different chromosomes would then determine the final value of a character. Additivity of groups of genes is the simpIest assumption for this model, but other methods of synergy can be easily interpreted in this way. It has, however, been shown that the action of genes affecting quantitative characters may depend on the genotypic environment, i.e. on other genes present in the same organism. In an organism with few chromosomes, such as Drosophila, it is possible to define the genotypic environment more closely by localizing the effects of the genotypic environment in one of the large chromosomes-the small fourth chromosome is usually neglected in studies of quantitative inheritance. The effect of the genic environment would then appear in the form of an interaction between different chromosomes. It is the purpose of the present paper to analyze the genetic determination of male body weight in four inbred strains of Drosophila melanogaster in terms of the action of the three major chromosomes. In studies of this type, it is difficult to differentiate between the contributions of the hereditary and environmental components of variance to the phenotype of the population. However, if one component of variance can be eliminated, the other can be estimated from the remaining variation of the phenotype. The use of isogenic strains effectively eliminates genotypic variation. Thus, a group of isogenic individuals raised under defined environmental conditions will exhibit a range of phenotypes which leads to an estimate of the environmental effects under these conditions. The effects obtained from one isogenic strain under specific environmental circumstances are peculiar to that strain. Other isogenic