Question

Based on the threshold range, at the X/A ratio of 0.67, the sex of {Drosophila will be:}

• A. Intersex
• B. Superfemale
• C. Supermale
• D. Male

Hint:

Memorize the key X/A ratio points for {Drosophila}: 1.0 is female, 0.5 is male. Anything between is intersex. Anything above 1.0 is superfemale, and below 0.5 is supermale. The Y chromosome in flies is important for male fertility but not for sex determination itself.

Answer 1

The Correct Option is A

Step 1: Concept Overview:
In *Drosophila melanogaster* (fruit flies), sex determination is governed by the Genic Balance Theory (Calvin Bridges). This theory states that the X/A ratio (number of X chromosomes to sets of autosomes) dictates sex.

Step 2: Key Ratio Thresholds:
The following X/A ratio thresholds define the sexual phenotype:

X/A Ratio \(\geq\) 1.0: Indicates a female (e.g., XX/2A = 1.0). Ratios exceeding 1.0 (e.g., XXX/2A = 1.5) result in a sterile, weak Superfemale (metafemale).
X/A Ratio = 0.5: Indicates a male (e.g., XY/2A = 0.5).
X/A Ratio<0.5: Results in a sterile, weak Supermale (metamale).
X/A Ratio between 0.5 and 1.0: Results in an Intersex, displaying mixed male and female traits.

Step 3: Explanation:
Given an X/A ratio of 0.67, we compare it to the established thresholds:
\[ 0.5<0.67<1.0 \] Because 0.67 falls between the male (0.5) and female (1.0) thresholds, the phenotype is Intersex. A fly with a 2X/3A chromosome configuration (\(\approx\) 0.67) exemplifies this ratio.

Step 4: Conclusion:
Therefore, an X/A ratio of 0.67 in *Drosophila* produces an Intersex phenotype, making option (A) correct.