Question

The formation of a loop during the pachytene stage of meiosis indicates occurrence of -
A: Translocation
B: Deficiency
C: Inversion
D: Duplication
Choose the correct answer from the options given below:

• A. A, B and D only
• B. A, B, and C only
• C. B, C, and D only
• D. A, C, and D only

Hint:

Loops are formed during synapsis to accommodate localized length mismatches (deficiency and duplication) or sequence orientation reversals (inversion), whereas translocations involve non-homologous exchanges and form cross-shaped structures.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
During the pachytene stage of meiotic prophase I, homologous chromosomes undergo side-by-side pairing (synapsis) mediated by the synaptonemal complex. For pairing to succeed, homologous gene sequences must align precisely. Structural changes in chromosomes alter this alignment.

Step 2: Detailed Explanation:
Let us analyze how each chromosomal aberration behaves during synapsis:
- B. Deficiency (Deletion): One chromosome is missing a segment. During synapsis, the normal chromosome must form a protruding deletion loop to allow the remaining homologous regions to pair.
- D. Duplication: One chromosome has an extra copy of a segment. To match the normal chromosome, the duplicated region on the modified chromosome must buckle out as a duplication loop.
- C. Inversion: A segment of a chromosome is reversed \( 180^{\circ} \). To allow homologous genes to pair in the correct orientation, both chromosomes must form a characteristic inversion loop.
- A. Translocation: Reciprocal translocations involve the exchange of segments between non-homologous chromosomes. During pairing, they form a cross-shaped quadrivalent structure (tetravalent) to maximize pairing of homologous segments, rather than a loop.
Therefore, loop formation during pachytene is characteristic of deficiency, inversion, and duplication.

Step 3: Final Answer:
Hence, the correct choice is B, C, and D only.