A child has blood group O. If the father has blood group A and mother blood group B, work out the genotypes of the parents and the possible genotypes of the other offsprings.

If a child has blood group O it has the genotype of “ii” which is recessive to both blood group A (I^A) and blood group B (I^B). As we know recessive traits only occur in homozygous condition thus both the parents must have the recessive allele making them heterozygous for their blood group. Thus the following punnett square can determine the possible genotypes and phenotypes of the other offsprings.