Drosophila eye morphology: The classic Bar eye phenotype in Drosophila melanogaster is primarily caused by which chromosomal change?

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Accepted Answer

Introduction / Context: The Bar eye phenotype (narrow, slit-like eyes) is a classic example used to illustrate gene dosage and chromosomal rearrangements in fruit flies. Understanding its cytogenetic basis helps connect phenotype to specific structural variations. Given Data / Assumptions: Phenotype: Bar (reduced eye size, slit-like appearance). Chromosomal region implicated: X chromosome, cytological band around 16A. Mechanism: copy number change (dosage effect). Concept / Approach: Bar results from a tandem duplication in the 16A region on the X chromosome. Increased gene dosage in this region reduces eye size. Further duplication (double Bar) exacerbates the phenotype, demonstrating dosage sensitivity rather than a simple point mutation or aneuploidy per se. Step-by-Step Solution: Identify the phenotype and associated locus (X, 16A).Recall that Bar is due to tandem duplication, not deletion.Understand dosage: more copies intensify Bar phenotype.Select “Duplication in region 16A of the X chromosome”. Verification / Alternative check: Classical cytogenetic studies and genetic mapping confirm increased copy number at 16A correlates with Bar and double Bar phenotypes. Why Other Options Are Wrong: Deletion: would typically reduce dosage; does not explain Bar. Extra X (XXY): affects sex determination and viability but is not the primary cause of Bar. Point mutation or translocation: not the canonical mechanism for Bar. Common Pitfalls: Confusing Bar (dosage/duplication) with white or sepia (point mutations) eye-color phenotypes controlled by single genes. Final Answer: Duplication in region 16A of the X chromosome

Animal Biotechnology Chromosomes Questions