Classical Drosophila genetics—cause of the ‘‘hairy wing’’ phenotype In fruit fly (Drosophila) genetics, the characteristic ‘‘hairy wing’’ trait is primarily caused by which chromosomal change?

Introduction / Context:
The ‘‘hairy wing’’ phenotype in Drosophila melanogaster is a classical example used in cytogenetics to illustrate how structural chromosome changes can alter gene dosage and produce visible traits. Understanding whether a phenotype results from duplication, deletion, or other rearrangements is a core learning goal in genetics.

Given Data / Assumptions:

• The phenotype involves excessive or ectopic trichomes (hairs) on the wing.
• The trait is historically associated with altered gene dosage rather than complete loss of function.
• We are comparing types of chromosome changes: duplication, deletion, aneuploidy (extra X), or point mutation.

Concept / Approach:
Gene duplications increase copy number of one or more genes. In Drosophila, several classic phenotypes (e.g., Bar eye, hairy wing) arose from segmental duplications that elevate expression of developmental genes, causing patterning changes. Deletions typically reduce structure or viability, extra sex chromosomes cause global sex-dosage effects, and point mutations often alter single-gene function without the characteristic cytological signature of a segmental dosage increase.

Step-by-Step Solution:

Verification / Alternative check:
Classical mapping and cytological studies in Drosophila associate hairy wing with duplicated chromosomal segments; restoring normal copy number suppresses the phenotype, consistent with a dosage mechanism.

Why Other Options Are Wrong:

• Deletion: typically reduces structures or causes lethality; does not classically produce hairy wing.
• Additional X chromosome: affects sex determination and viability, not specifically wing trichome pattern.
• Point mutation only: possible in principle, but the textbook association for hairy wing is a duplication event.
• Pericentric inversion: reorders segments but does not inherently change dosage.

Common Pitfalls:
Confusing any visible trait with a point mutation. Many classic Drosophila markers reflect copy-number changes rather than simple base substitutions.

Final Answer:
Duplication in a region of a chromosome