Question

Assertion (A) : The human blood group ‘AB’ exemplifies co-dominance.

Reason (R) : The gene for human blood group trait exists in three allelic forms \( I^{A} \), \( I^{B} \) and i.

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

Hint:

AB shows co-dominance and ABO has three alleles, but multiple alleles do not explain co-dominance.

Answer 1

The Correct Option is B

Step 1: Recall ABO blood groups:
The ABO gene has three alleles: \( I^{A} \), \( I^{B} \) and \( i \). \( I^{A} \) makes antigen A, \( I^{B} \) makes antigen B and \( i \) makes none.

Step 2: Check the Assertion:
A person with genotype \( I^{A}I^{B} \) shows both A and B antigens on red cells. Both alleles express fully. This is co-dominance, so (A) is true.

Step 3: Check the Reason:
The ABO gene indeed exists in three allelic forms \( I^{A} \), \( I^{B} \) and \( i \). So (R) is true.

Step 4: Link R to A:
Co-dominance arises because \( I^{A} \) and \( I^{B} \) are both expressed in the heterozygote. Having three alleles (multiple alleles) does not by itself explain this. A gene with three alleles could show only complete dominance. So R is not the explanation.

Step 5: Pick the code:
Both are true but R does not explain A. This is code (B), the second option.

Final Answer:
Both A and R are true, but R is not the correct explanation of A.
\[ \boxed{\text{Option (2)}} \]