(b) : The phenomenon of expression of both the alleles in a heterozygote is called codominance. The alleles which do not show dominance­recessive relationship 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 $A\ (I^A)$ and for blood group $B\ (I^B)$ are codominant so that when they come together in an individual, they produce blood group $AB$.