Q.14 Determine the correctness or otherwise of the following Assertion [a] and the
Reason [r].
Assertion [a]: Chromosome mutations can change the structure of chromosomes.
Reason [r]: All chromosome mutations arise due to nondisjunction of
chromosomes during mitosis or meiosis.
(A) Both [a] and [r] are false
(B) [a] is true but [r] is false
(C) Both [a] and [r] are true and [r] is the correct reason for [a]
(D) Both [a] and [r] are true but [r] is not the correct reason for [a]

Chromosome mutations alter chromosome architecture through breakage and faulty rejoining, while nondisjunction primarily affects chromosome numbers. The assertion holds true, but the reason does not fully explain it, making option B correct. This analysis breaks down the GATE BT 2023 question for competitive exam preparation like IIT JAM.

Assertion Evaluation

The assertion states that chromosome mutations can change chromosome structure, which is accurate. These mutations include deletions (segment loss), duplications (segment repetition), inversions (segment reversal), and translocations (segment exchange between non-homologous chromosomes), all arising from chromosome breakage and improper repair. Such structural changes disrupt gene order, dosage, and function without altering chromosome count.

Reason Analysis

The reason claims all chromosome mutations arise from nondisjunction during mitosis or meiosis, which is false. Nondisjunction occurs when chromosomes fail to separate properly, leading to numerical abnormalities like aneuploidy (e.g., trisomy in Down syndrome) rather than structural alterations. Structural chromosome mutations typically result from DNA breaks induced by radiation, chemicals, or replication errors, not nondisjunction.

Option Breakdown

• (A) Both false: Incorrect, as assertion [a] is true—structural changes like inversions and translocations directly modify chromosome morphology.

• (B) [a] true, [r] false: Correct. Assertion holds due to breakage-rejoining mechanisms; reason fails because nondisjunction causes numerical issues (e.g., extra/missing chromosomes), not structural ones.

• (C) Both true, [r] explains [a]: Wrong—reason [r] is false, so it cannot justify the assertion.

• (D) Both true, [r] not explanation: Incorrect since reason [r] is false; nondisjunction does not produce all structural mutations.

Exam Relevance

This question tests distinction between structural (e.g., translocations) and numerical (e.g., nondisjunction) chromosomal aberrations, key for IIT JAM Biotechnology and GATE BT syllabi on genetics. Practice recognizing mechanisms: breakage for structure, segregation errors for number.