Practice question

Bar eyes in Drosophila melanogaster were first described by Tice and studied by Sturtevant and Bridges as classic tandem duplication. The X-chromosome region 16A containing Bar gene duplicates, producing B, double-Bar, and ultra-Bar alleles. Increased copy number reduces facet number through position effect and dosage, creating narrow slit eyes instead of round wild-type eyes. Unequal crossing over between homologous repeats expands or contracts copy number, demonstrating dynamic nature of tandem duplications. It became textbook proof that phenotype can arise from dosage, not just mutation in coding sequence.