Figure 1.

Bar alleles and their phenotypes. (A) Wild-type Drosophila have round eyes. Flies homozygous (or hemizygous) for the Bar mutation have thin, slit-like eyes. Flies homozygous (or hemizygous) for the double-Bar mutation have even smaller eyes. (B) Schematic of the Bar region of polytene chromosomes. The Bar mutation is a tandem duplication and double-Bar a tandem triplication of the region. (C) The Bar mutation arose by unequal crossing over between two Roo transposable elements (yellow), resulting in a tandem duplication. Reversion and triplication alleles arose from the Bar mutant by unequal crossover between duplicates that had aligned out of register.