The Interaction of Mutants Affecting Venation in Drosophila Melanogaster. I. Interaction of Hairless, Engrailed and Cubitus Interruptus.

N E approach to the problem of how genes manifest themselves in develop0 ment has involved the analysis of the effects of changes at a given gene locus, such changes taking the form of alterations in the combination, dosage and position of alleles. Except where the analysis of modifier action is involved, the emphasis of this type of study is on the effects of a disturbance in the normal genotypic complement at a single point. While this type of study has the advantage of dealing with relatively few genetic variables, it has the disadvantage of excluding from analysis the contributory action of the remainder of the genotype to the production of normality in the specific phenotypic character under investigation. The latter problem is perhaps better attacked by means of genetic interaction studies which make use of disturbances at Several points in the genotypic system, analyzed singly and in combination. Such genetic interaction studies have probably found greatest success in investigations dealing with the development of pigmentation in organisms. The work of WRIGHT (1941) on the interaction of the major color factors in the production of pigmentation in the coat of the guinea pig, the work of LAWRENCE and SCOTT-MONCRIEFF (1935) on gene interaction in the determination of floyer color, and the work of BEADLE and EPHRUSSI (1936) on the genetic control of eye color in Drosophila are perhaps the most completely analyzed of such studies. Among other investigations primarily devoted to the study of gene interaction may be mentioned the work of NEEL (1941) and KARP (1936) on the interaction of loci concerned with bristle-forming reactions in Drosophila; that of CSIK (1934), SEME~TZA (1951), and WALETSKY (1939) on the interaction of mutants affecting the Drosophila wing ; the investigations of MAINX (1938) on gene interactions affecting eye color ; the work of SCHULTZ (1939) on interactions dealing with the minute phenotype ; and the study of CLARK (1949) on the interaction of loci affecting wing size in Habrobracon. The system of veins in the Drosophila wing affords an unusually convenient phenotypic system for an interaction analysis. The relative simplicity of the pattern of longitudinal and cross veins, the precise manner in which the effects of various mutant substitutions on this pattern may be ascertained, and the large number of mutants available which produce disturbances in this pattern