The Relation of a Dominant Eye Color in Drosophila Melanogaster to the Associated Chromosome Rearrangement.

The possibility that the developmental effects of genes are influenced by their neighbors in the chromosomes has only recently become accessible to experiment. The first demonstration of such an effect was given by STURTEVANT (1925). Two Bar genes in Drosophila melanogaster, placed in the same chromosome as a result of unequal crossing over, are more effective than two similar genes present in the two chromosomes of a homozygote. The expression of the Bar gene is dependent upon its position in the chromosome. It was early evident (STURTEVANT 1925, p, 138) that chromosome rearrangements should provide further data for the statement and solution of the problem. Following a chromosome rearrangement, the genes adjoining the break change neighbors. If the behavior of a gene is a function of its relations to its neighbors, a t the points of breakage in chromosome rearrangements, changes resembling mutations should be found. In point of fact, the original “Pale-translocation” was detected by virtue of a change inseparable from the locus of the translocation (BRIDGES 1923, BRIDGES and MORGAN 1923). With the advent of the X-ray technique for producing chromosome rearrangements on a large scale, the problem was really opened to attack. In the X-ray translocations, MULLER and ALTENBURG (1930) and DOBZHANSKY (1930) have shown that most translocations in Drosophila melanogaster contain lethal “mutations” a t the point of fracture. And, as is usual in the relation between lethal and non-lethal mutants, where many of the former are found, some of the latter also occur. A few cases involving known mutants at the breaks have been studied in detail. DOBZHANSKY (1932) analyzed a translocation in which the X chromosome was broken a t the Bar locus, the second chromosome a t vestigial, and an interchange effected. At the point of translocation, in place of the wild-type section present in the treated chromosome, a new allele of Bar was found. The second mutation from wild-type to Bar, in all the ’ CARNEGIE INSTITUTION of WASHINGTON.