Assertion–Reason Questions

🧬 Mutation | Class 12 Biology

Directions:

Choose the correct option:

A. Both Assertion (A) and Reason (R) are true and R is the correct explanation of A
B. Both A and R are true but R is not the correct explanation of A
C. A is true but R is false
D. A is false but R is true

Question 1

Assertion (A): Mutation is the ultimate source of genetic variation.

Reason (R): Mutation creates new alleles in a population.

✅ Correct Option: A

Explanation: New alleles arise only through mutation, causing variation.

Question 2

Assertion (A): Sickle cell anaemia is caused by a point mutation.

Reason (R): A single base substitution changes glutamic acid to valine.

✅ Correct Option: A

Explanation: Single nucleotide change alters amino acid.

Question 3

Assertion (A): Frameshift mutations are more harmful than substitution mutations.

Reason (R): They alter the reading frame of codons.

✅ Correct Option: A

Explanation: Entire protein structure changes.

Question 4

Assertion (A): Silent mutations do not change the amino acid sequence.

Reason (R): Genetic code is degenerate.

✅ Correct Option: A

Explanation: Multiple codons code for same amino acid.

Question 5

Assertion (A): Chromosomal mutations involve changes in chromosome structure.

Reason (R): They include deletion, duplication, inversion and translocation.

✅ Correct Option: A

Question 6

Assertion (A): Polyploidy is common in plants but rare in animals.

Reason (R): Animals cannot tolerate change in chromosome number.

❌ Correct Option: B

Explanation: Reason is true but not the complete explanation.

Question 7

Assertion (A): Down syndrome is an example of aneuploidy.

Reason (R): It is caused by trisomy of chromosome 21.

✅ Correct Option: A

Question 8

Assertion (A): Germinal mutations contribute to evolution.

Reason (R): They are inherited by the next generation.

✅ Correct Option: A

Question 9

Assertion (A): Somatic mutations are not passed to offspring.

Reason (R): They occur in body cells.

✅ Correct Option: A

Question 10

Assertion (A): Mutagens increase the rate of mutation.

Reason (R): They cause changes in DNA structure or sequence.

✅ Correct Option: A

Question 11

Assertion (A): UV radiation causes thymine dimers.

Reason (R): UV rays cause bonding between adjacent thymine bases.

✅ Correct Option: A

Question 12

Assertion (A): Nitrous acid acts as a chemical mutagen.

Reason (R): It causes deamination of nitrogenous bases.

✅ Correct Option: A

Question 13

Assertion (A): Inversion mutation does not change gene number.

Reason (R): It only reverses the orientation of chromosome segment.

✅ Correct Option: A

Question 14

Assertion (A): Duplication mutation increases gene dosage.

Reason (R): A chromosomal segment is repeated.

✅ Correct Option: A

Question 15

Assertion (A): Translocation mutation involves non-homologous chromosomes.

Reason (R): Chromosomal segments are exchanged between different chromosomes.

✅ Correct Option: A

Question 16

Assertion (A): Aneuploidy results from nondisjunction.

Reason (R): Chromosomes fail to separate during cell division.

✅ Correct Option: A

Question 17

Assertion (A): Polyploidy leads to larger cell size in plants.

Reason (R): Increased chromosome sets increase nuclear content.

✅ Correct Option: A

Question 18

Assertion (A): Cancer is associated with somatic mutations.

Reason (R): These mutations cause uncontrolled cell division.

✅ Correct Option: A

Question 19

Assertion (A): All mutations are harmful.

Reason (R): Mutations disturb normal genetic balance.

❌ Correct Option: D

Explanation: Many mutations are neutral or beneficial.

Question 20

Assertion (A): Mutation alone cannot cause evolution.

Reason (R): Natural selection is required to act on variation.

✅ Correct Option: A