Question

Occurrence of unreduced gametes (i.e. 2n) has been reported in several crops including potatoes. Unreduced gametes can be produced by -
(A) First-division restitution
(B) Parallel spindle in the second meiotic division
(C) Second division restitution
(D) Occurrence of third meiotic division

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

Hint:

In potato breeding, unreduced ($2n$) gametes are highly valued because they allow breeders to cross diploid wild species directly with tetraploid cultivated varieties ($4x \times 2x \rightarrow 4x$).

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
In sexual reproduction, meiosis normally reduces the chromosome number by half, producing haploid ($1n$) gametes.
However, meiotic mutations or spindle abnormalities can prevent this reduction, producing unreduced ($2n$) gametes.

Step 2: Detailed Explanation:
Let us analyze the mechanisms that produce unreduced ($2n$) gametes:
- First-Division Restitution (FDR) (A): This occurs when the first meiotic division fails (e.g., due to lack of homologous pairing or spindle failure).
The chromosomes do not separate into two nuclei, and the cell proceeds directly to the second division, producing gametes that contain the complete somatic chromosome complement. (True)
- Second-Division Restitution (SDR) (C): This occurs when the first meiotic division proceeds normally, but the second division fails (e.g., due to lack of chromatid separation or cytokinesis failure), producing unreduced gametes. (True)
- Parallel Spindle in the Second Meiotic Division (B): During normal metaphase II, the two spindles are oriented perpendicular or tilted relative to each other.
If the spindles align parallel to each other, the chromatids separate into a single plane during anaphase II. This results in the formation of a dyad containing two unreduced $2n$ gametes instead of a tetrad. (True)
- Third Meiotic Division (D): Meiosis consists strictly of two divisions (meiosis I and II). There is no "third meiotic division" in any biological system.
Therefore, only mechanisms (A), (B), and (C) can produce unreduced gametes.

Step 3: Final Answer:
The correct mechanisms are (A), (B), and (C) only, corresponding to option (A).