Assertion (A): Mutation is a phenomenon which results in alteration of DNA sequences and consequently results in changes in the genotype and the phenotype of an organism.
Reason (R): A point mutation is a change of a single base pair in DNA.
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → A and R both are true but R is not the correct explanation of A.
Assertion (A): Mutation is a phenomenon which results in alteration of DNA sequences and consequently results in changes in the genotype and the phenotype of an organism. — True. This is the book’s definition of mutation. When the DNA sequence of a gene changes, the genotype changes, and the protein made from that gene may change, so the phenotype can change too.
Reason (R): A point mutation is a change of a single base pair in DNA. — True. A change of one base pair in DNA is a point mutation. Sickle-cell anaemia is the classic example.
Why R does not explain A? A is about what mutation in general is and what it leads to: a change in the DNA sequence, and so in genotype and phenotype. R only names one kind of mutation. Mutations also arise from loss or gain of a segment of DNA and from changes in whole chromosomes, and these too change the genotype. R gives a type of mutation, not the reason why a change in DNA alters the genotype and phenotype. So both statements are correct, but R is not the explanation of A.
NCERT Extract:
Mutation – “Mutation is a phenomenon which results in alteration of DNA sequences and consequently results in changes in the genotype and the phenotype of an organism. In addition to recombination, mutation is another phenomenon that leads to variation in DNA. … In addition to the above, mutation also arise due to change in a single base pair of DNA. This is known as point mutation. A classical example of such a mutation is sickle cell anemia.”