A sex-linked dominant mutation in Drosophila melanogaster which results in a reduction in the number of facets in the compound eye. This mutation, symbolized by B, is commonly used as a marker when constructing balancer X chromosomes. The original mutation contained a tandem duplication of chromosome segment 16A. Unequal crossing over (q.v.) can cause this mutation to revert to wild type. The 16A duplication is now thought to be a transposon-induced rearrangement. Analysis of the Bar phenomenon led to the discovery of position effects (q.v.). See Chronology, 1925, Sturtevant; M5 technique, transposable elements.
Subjects: Genetics and Genomics.