Question

Given below are two statements:
Statement (I): Drosophila flies with 3X: 3A chromosomes are normal females.
Statement (II): Drosophila flies with 3X: 2A chromosomes are metafemales.
In light of the above statements, choose the most appropriate answer from the options given below:

• A. Both Statement (I) and Statement (II) are correct.
• B. Both Statement (I) and Statement (II) are incorrect.
• C. Statement (I) is correct but Statement (II) is incorrect.
• D. Statement (I) is incorrect but Statement (II) is correct.

Hint:

Remember the core values of Bridges' genic balance theory:
Ratio = 1.0 \(\rightarrow\) Female.
Ratio \( > \) 1.0 \(\rightarrow\) Metafemale.
Ratio = 0.5 \(\rightarrow\) Male.
Ratio \( < \) 0.5 \(\rightarrow\) Metamale.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Calvin Bridges proposed the genic balance theory of sex determination in *Drosophila melanogaster*.
This theory states that sex is determined by the ratio of the number of X chromosomes (X) to the number of haploid sets of autosomes (A).

Step 2: Detailed Explanation:
According to the genic balance theory, we calculate the X:A ratio to determine the sexual phenotype of the fly:
If the X:A ratio is exactly \( 1.0 \), the fly develops as a female.
If the X:A ratio is greater than \( 1.0 \), the fly develops as a metafemale (or superfemale).
If the X:A ratio is exactly \( 0.5 \), the fly develops as a male.
If the X:A ratio is between \( 0.5 \) and \( 1.0 \), the fly develops as an intersex.
Let us evaluate the two statements:
Statement (I): Drosophila with 3X: 3A.
The ratio is:
\[ \frac{X}{A} = \frac{3}{3} = 1.0 \] Since the ratio is \( 1.0 \), these triploid flies develop as normal, fertile females.
Thus, Statement (I) is correct.
Statement (II): Drosophila with 3X: 2A.
The ratio is:
\[ \frac{X}{A} = \frac{3}{2} = 1.5 \] Since the ratio is greater than \( 1.0 \), these flies develop as metafemales, which are usually weak and sterile.
Thus, Statement (II) is correct.

Step 3: Final Answer:
Both Statement (I) and Statement (II) are correct.