Question

Given below are two statements:
Statement (I): In usual terms, recombination frequency between any pair of genes does not exceed 50%.
Statement (II): In a situation of 4-strands double cross-over, recombination frequency becomes 100%.
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:

While the overall recombination frequency between two genes on a chromosome cannot exceed \( 50% \), the recombination frequency within a single tetrad that undergoes a 4-strand double crossover is \( 100% \).

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Recombination frequency is a measure of genetic linkage, representing the percentage of recombinant offspring produced during a genetic cross.
It is determined by the frequency of physical crossing-over events between homologous chromosomes during prophase I of meiosis.

Step 2: Detailed Explanation:
Let us analyze both statements:
1. Analysis of Statement (I): During meiosis, crossing over occurs at the four-strand stage, involving only two of the four chromatids in a homologous pair.
Even if a crossover occurs between two genes in every single meiosis, only two of the four resulting gametes will be recombinant, yielding a maximum recombination frequency of \( 50% \).
Multiple crossovers (such as double crossovers) can occur, but across all meiotic events, they average out such that the recombination frequency between any two loci asymptotically approaches \( 50% \).
Therefore, Statement (I) is correct.
2. Analysis of Statement (II): A double crossover can involve two, three, or four of the chromatids in a homologous pair:
- 2-strand double crossover: Involves two chromatids, yielding \( 0% \) recombinants (all four chromatids revert to parental configurations).
- 3-strand double crossover: Involves three chromatids, yielding \( 50% \) recombinants.
- 4-strand double crossover: Involves all four chromatids.
In a 4-strand double crossover, both crossovers occur such that all four chromatids undergo a reciprocal exchange.
As a result, all four resulting gametes from that specific meiotic event carry recombinant chromosomes.
Therefore, for that specific tetrad, the recombination frequency is indeed \( 100% \), making Statement (II) correct.

Step 3: Final Answer:
Both Statement (I) and Statement (II) are correct, which corresponds to option (A).