Developmental genetics — Drosophila sex determination: When is the Sex-lethal (sxl) gene transcribed with respect to the X:autosome (X:A) balance?

Introduction / Context:
In Drosophila melanogaster, primary sex determination depends on the X chromosome to autosome ratio (X:A). The master regulator Sex-lethal (sxl) initiates the female-specific splicing cascade when the X:A balance indicates a female genotype.

Given Data / Assumptions:

• X:A = 1.0 (e.g., XX with two autosome sets) corresponds to female.
• X:A = 0.5 (e.g., XY with two autosome sets) corresponds to male.
• sxl early promoter activity reflects numerator/denominator gene balance.

Concept / Approach:
In embryos with X:A ≥ 1.0, transcription from the early sxl promoter is initiated, leading to production of Sxl protein that enforces female-specific splicing of transformer (tra) and doublesex (dsx) transcripts. In embryos with X:A = 0.5, sxl is not activated, and male development proceeds.

Step-by-Step Solution:

Verification / Alternative check:
Genetic mosaics and mutations in numerator (X-linked) and denominator (autosomal) elements shift sxl activation in predictable ways, validating the ratio mechanism.

Why Other Options Are Wrong:

• X:A < 0.5 or = 0.5 correspond to male pathway; sxl remains OFF.
• “None” is incorrect because a definitive rule exists.

Common Pitfalls:
Confusing Drosophila ratio-based sex determination with mammalian SRY-based determination; mechanisms differ fundamentally.

Final Answer:
When the X:A ratio is 1.0 or greater