Question:
Two genes say A and B, are sex-linked genes in Drosophila melanogaster. Which of the following statements about the genes are correct ?
(A) A and B may be linked
(B) A and B may also have a 50% recombination frequency
(C) A and B are linked and can recombine only in males
(D) A and B are located in the same linkage group
Two genes say A and B, are sex-linked genes in Drosophila melanogaster. Which of the following statements about the genes are correct ?
(A) A and B may be linked
(B) A and B may also have a 50% recombination frequency
(C) A and B are linked and can recombine only in males
(D) A and B are located in the same linkage group
(A) A and B may be linked
(B) A and B may also have a 50% recombination frequency
(C) A and B are linked and can recombine only in males
(D) A and B are located in the same linkage group
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The complete absence of crossing over in male *Drosophila* simplifies genetic mapping crosses, allowing researchers to track intact paternal chromosomes without recombinant variations.
Updated On: Jul 23, 2026
- (A), (B) and (C) only.
- (B), (C) and (D) only.
- (A), (B), (C) and (D).
- (A), (B) and (D) only.
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The Correct Option is D
Solution and Explanation
Step 1: Understanding the Concept:
Sex-linked genes are located on the sex chromosomes (specifically the X chromosome in *Drosophila*).
Linkage groups correspond to physical chromosomes, and genetic recombination behaves differently in male and female *Drosophila*.
Step 2: Detailed Explanation:
Let us evaluate each of the statements:
- Statement (A) and (D): Since both genes A and B are sex-linked, they are located on the same X chromosome.
Genes located on the same physical chromosome are physically linked and belong to the same linkage group.
Therefore, statements (A) and (D) are correct.
- Statement (B): If two genes on the same chromosome are located far apart (e.g., at opposite ends of the chromosome), crossing over will occur between them in almost every meiosis.
This results in a maximum recombination frequency of $50\%$, making them appear genetically unlinked despite being physically on the same chromosome.
Therefore, statement (B) is correct.
- Statement (C): In *Drosophila melanogaster*, male flies do not undergo meiotic crossing over (achiasmatic meiosis).
Genetic recombination and crossing over occur exclusively in females.
Therefore, the statement that they "can recombine only in males" is incorrect.
Thus, only statements (A), (B), and (D) are correct.
Step 3: Final Answer:
The correct statements are (A), (B), and (D), which corresponds to option (D).
Sex-linked genes are located on the sex chromosomes (specifically the X chromosome in *Drosophila*).
Linkage groups correspond to physical chromosomes, and genetic recombination behaves differently in male and female *Drosophila*.
Step 2: Detailed Explanation:
Let us evaluate each of the statements:
- Statement (A) and (D): Since both genes A and B are sex-linked, they are located on the same X chromosome.
Genes located on the same physical chromosome are physically linked and belong to the same linkage group.
Therefore, statements (A) and (D) are correct.
- Statement (B): If two genes on the same chromosome are located far apart (e.g., at opposite ends of the chromosome), crossing over will occur between them in almost every meiosis.
This results in a maximum recombination frequency of $50\%$, making them appear genetically unlinked despite being physically on the same chromosome.
Therefore, statement (B) is correct.
- Statement (C): In *Drosophila melanogaster*, male flies do not undergo meiotic crossing over (achiasmatic meiosis).
Genetic recombination and crossing over occur exclusively in females.
Therefore, the statement that they "can recombine only in males" is incorrect.
Thus, only statements (A), (B), and (D) are correct.
Step 3: Final Answer:
The correct statements are (A), (B), and (D), which corresponds to option (D).
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