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O N E of the major directions of research bearing on the organization of the genetic material was initiated with the first reports of exceptional segregants among the progeny of females of Drosophila melanogaster heterozygous for two different mutant alleles of a given gene, and the association of these exceptions with crossing over (OLIVER 1940; LEWIS 1941). These reports led to extensive reexamination of multiple allelic series in higher organisms (largely Drosophila), with particular emphasis upon loci which showed signs of functional complexity as exhibited by pleiotropy and unusual phenotypic interactions in heterozygotes. Such studies usually were successful in subdividing a series of alleles into some small number of recombinationally separable units, termed pseudoalleles. From these early studies there emerged the notion of a pseudoallelic locus as consisting of a small number of genetic units whose recombinational distinction was paralleled by functional distinction, and whose phenotypic interactions indicated some relationship between their separable functions (see reviews by CARLSON 1959; GREEN 1963). Parallel studies with microorganisms uncovered similarly close linkage between genetic units which control enzymes governing physiologically related reactions. Moreover, elaborate evidence is available from such microbial studies which provides an understanding of the coordinated regulation of the function of these linked systems now termed operons (see reviews by AMES and HARTMAN 1963; JACOB and MONOD 1961). The existence of such gene control systems in higher organisms has been known for some time in maize (MCCLINTOCK 1961) and consideration has been given to their possible significance for questions of development in higher organisms (LEWIS 1963; JACOB and MONOD 1963). Turning next to a single genetic functional unit or cistron (BENZER 1957), the microbial evidence indicates that a typical cistron possesses numerous, separable, linearly ordered sites capable of undergoing independent mutation ( BENZER 1959, 1961). Experimental analysis of functional organization within a single cistron presents a compelling argument relating the linear order of recombinational sites within a cistron to the genetic code for proteins (CRICK 1963; CRICK,

THE ROSY CISTRON IN DROSOPHILA MELANOGASTER: GENETIC FINE STRUCTURE ANALYSIS