When crossing two plants homozygous for contrasting pair of characteristics results in a hybrid showing complete expression of both phenotypes is called as codominance. It is another deviation from Mendel's law of dominance. For example, when blood type A (homozygous) and blood type B (homozygous) are crossed, the resultant offspring has blood type AB where both allele for A and allele for B are expressed completely without any interference from each other.

Blood group O is expressed in homozygous recessive condition - ii and the child has blood group O.

Each parent provides one gene each for expression of the blood group, thus, each parent has the allele 'i'in their genotype. Father is having blood group A and the genotype will be ${\mathrm{I}}^{\mathrm{A}}\mathrm{i}$ and similarly for mother the genotype with blood group B will be ${\mathrm{I}}^{\mathrm{B}}\mathrm{i}$.