A child has blood group O. If the father has blood group A and mother blood group B

Blood group in human is controlled by three kinds of alleles I^A, I^B and i. I^A and I^B are codominant whereas I is recessive. I^A codes for blood group A whereas I^B codes for blood group B. O blood group is coded by the i allele. A person with blood group A can have two possible allele combinations- I^A I^A or I^A i. A person with blood group B can also have two combinations I^B I^B or I^B i When both I^A and I^B are present then the blood group will be AB and when both of these are absent then the blood group will be O (ii) In the above case since the father and mother have dominant alleles and yet the child shows the recessive phenotype, the dominant A and B alleles in the parent must be present in the heterozygous condition. This is because if the alleles were present in homozygous condition in any of the parent then the child would show dominant phenotype and not the recessive phenotype. Therefore, we can see that in the above situation the other offspring could show A, B or AB blood group.