Q.58 Which two of the following are the CORRECT statements ?
P. Nondisjunction in the parental meiosis is not essential to produce diploid organisms
Q. Nondisjunction in the parental meiosis is essential to produce aneuploid organisms
R. Nondisjunction in the parental meiosis is essential to produce hexaploid organisms
S. Nondisjunction in the parental meiosis is essential to produce tetraploid organisms
(A) P,Q (B) Q,R (C) R,S (D) P,S
Nondisjunction in Meiosis: Correct Statements Identified
Nondisjunction—failure of chromosomes to separate during meiosis—produces gametes with abnormal chromosome numbers, leading to aneuploid or polyploid offspring upon fertilization. Q.58 asks which two statements about its role in producing diploids, aneuploids, and polyploids are correct.
Correct Answer: (A) P, Q
P and Q are true: Nondisjunction in parental meiosis is NOT essential for diploid organisms but IS essential for aneuploid organisms.
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P: Diploid organisms (2n) arise normally from haploid gametes (n + n = 2n). Nondisjunction can produce diploids (e.g., unreduced 2n gametes via meiosis I failure), but it’s not required—standard meiosis suffices.
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Q: Aneuploids (e.g., trisomy 2n+1, monosomy 2n-1) REQUIRE nondisjunction in parental meiosis I/II, yielding n+1 or n-1 gametes that form unbalanced zygotes upon fusion.
Why R and S Are Incorrect
Polyploidy (e.g., tetraploid 4n, hexaploid 6n) typically occurs via mechanisms bypassing nondisjunction.
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R: Hexaploids (e.g., wheat 6n=42) form by allopolyploidy—hybridization of diploids followed by chromosome doubling (spontaneous or colchicine-induced). Parental meiosis nondisjunction unnecessary.
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S: Tetraploids (4n) arise from diploid selfing with unreduced gametes, somatic doubling, or failure of meiosis/cytokinesis—not strictly “essential” nondisjunction in parental meiosis.
Statement Validity Table
| Statement | Ploidy Type | Requires Parental Nondisjunction? | Explanation |
|---|---|---|---|
| P | Diploid (2n) | No | Normal meiosis produces diploids |
| Q | Aneuploid | Yes | n+1/n-1 gametes only from nondisjunction |
| R | Hexaploid (6n) | No | Allopolyploidy + doubling |
| S | Tetraploid (4n) | No | Somatic doubling common |
Key Examples
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Aneuploid: Down syndrome (trisomy 21) from maternal meiosis I nondisjunction.
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Polyploid: Seedless watermelon (triploid 3n) from 2n × n gametes; no nondisjunction needed.
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Exam tip: Distinguish aneuploidy (unbalanced, nondisjunction) from euploid polyploidy (balanced multiples).
Master this for genetics questions—focus on “essential” vs. “possible.”
