Question

Given below are two statements:
Statement I : The crossing-over takes place at 4-strand stage of the chromosome
Statement II : Recombinant frequency usually does not exceed 50%
In light of the above statements, choose the most appropriate answer from the options given below:

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

Hint:

Crossing over occurs at the 4-strand tetrad stage (pachytene); Maximum recombination frequency between linked genes is $50\%$.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Cytological and tetrad analysis (e.g., in Neurospora) demonstrates the exact meiotic stage and recombination kinetics of crossing-over.

Step 2: Detailed Explanation:
1. Statement I is TRUE: Crossing-over occurs between non-sister chromatids of paired homologous chromosomes during the pachytene stage of Prophase I, when each chromosome is replicated into two chromatids, forming a four-strand tetrad structure.
2. Statement II is TRUE: Because each single crossover event involves only two of the four chromatids (leaving two non-recombinant parental strands), the maximum theoretical and observed recombination frequency between two linked loci cannot exceed $50\%$.

Step 3: Final Answer:
Therefore, both Statement I and Statement II are true.