The child with blood group O will have homozygous recessive alleles. Therefore, both the parents should be heterozygous, i.e. genotype of father will be ${{I}^{A}}i or {{I}^{B}} i and of mother will be {{I}^{A}} i or {{I}^{B}} i. A child have blood group O in the following two cases: Case I When father is {{I}^{A}}i and mother is {{I}^{B}}$ i.
The offsprings will have the above possible blood groups, i.e. AB, A, B and O.
Case II When father is {{I}^{B}} i and mother is {{I}^{A}} i.
