Position Effect and Gene Divisibility Considered in Connection with Three Strikingly Similar Scute Mutations.

FTER the proposal of the subgene theory by SEREBROVSKY, DUA BININ and their co-workers (SEREBROVSKY and DUBININ, 1929 et seq.), in interpretation of the relationships observed by them among scute alleles, an attempt was made by MULLER and co,-workers, first at the University of Texas laboratory and later at the Institute of Genetics of the U.S.S.R., to obtain further scute alleles and to analyse these and the ones previously obtained elsewhere, with a view to further testing the above theory and to further investigation of the curious relationship which he had observed between “gene mutations’’ and “gene iearrangements” (MULLER 1930a, b, 1932). It was soon evident that a high proportion of the scute mutations involved the breakage and reattachment of the X chromosome very close to the locus of scute, but i t was not clear whether the phenotypic change represented a “gene mutation” that was near to but, theoretically a t least, separable from the rearrangement, or whether it represented the “position effect” of the gene rearrangement itself. We shall not describe here the work of analysing the position of breakage of these rearrangements of the scute region with respect to each other, reported by MULLER and PROKOFYEVA (1934, 1935a) and briefly mentioned below (p. 566). Suffice i t here to say that the numerous rearrangements, although all appearing to possess one point of breakage and interchange of gene connections in one of four definite positions near scute had their other point of breakage anywhere in the X or other chromosomes, so that the gene arrangements occurring in the neighborhood of scute, after the breakages and reattachments had taken place, were in all cases very different from those in the normal chromosome and (with the exceptions to be noted presently) from each other. Correspondingly there was very much diversity in the phenotypic expression of the ‘alleles,’ although no fixed relation was discernible between the map position of the different