Question

Sol.(b) [Key Concept: Codominance — AB Blood Group] A person with I^A I^B genotype shows blood group AB because of CODOMINANCE. Both I^A and I^B alleles are fully and independently expressed simultaneously in the heterozygote — I^A directs production of A antigens and I^B directs production of B antigens on the same red blood cell. Neither allele is dominant over the other; both are expressed equally. This is the defining feature of codominance — no new intermediate phenotype, both parental phenotypes expressed fully.