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

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:

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