ABO blood grouping is controlled by gene I which has three alleles and show co-dominance. There are six genotypes. How many phenotypes in all are possible?

Human blood group inheritance is the example of codominance and multiple alleles. It is governed by three alleles namely IA, IB and i. IA and IB show codominance while allele "i" is recessive to both "IA" and "IB". This gives total six genotypes and four phenotypes- A (IAIA and IAi), B (IBIB and IBi), AB (IAIB) and O (ii). Thus, the correct answer is option D.