The phenomenon of expression of both the alleles in a heterozygote is called codominance. The alleles which do not show dominance-recessiverelationship and are able to express themselves independently when present together are called codominant alleles. As a result the heterozygous condition has a phenotype different from either of homozygous genotypes, e,g. alleles for blood group A (I4) and for blood group B(JB) are codominant so that when they come together in an individual, they produce blood group AB.