70. Given below are two statements: one is labelled as Assertion A and the other is labelled as Reason
R
Assertion A: A base substitution in DNA does not always result in mutant phenotype.
Reason R: Some base substitutions in codons do not change the encoded amino acid.
In the light of the above statements, choose the most appropriate answer from the options given
below
1. Both A and R are correct and R is the correct explanation of A
2. Both A and R are correct but R is NOT the correct explanation of A
3. A is correct but R is not correct
4. A is not correct but R is correct

 db

Detailed Explanation:

Assertion A: A base substitution in DNA does not always result in mutant phenotype.
True.
Many base substitutions are silent mutations, meaning they don’t change the amino acid sequence due to the redundancy of the genetic code.

Reason R: Some base substitutions in codons do not change the encoded amino acid.
True.
This is a direct explanation of the assertion. Multiple codons can code for the same amino acid (a phenomenon called codon degeneracy), so a change in one base might still result in the same amino acid.

Conclusion:

Both statements are correct, and Reason R correctly explains Assertion A. This reflects the biological principle of silent mutations which often occur in the third base of a codon — the “wobble” position.