Codominance is a form of dominance in which the alleles of a pair of genes in a heterozygotic condition are fully expressed resulting in a phenotype which is neither recessive nor dominant. When alleles show codominance, the characteristics of both are expressed in the phenotype.
Example of Codominance
A good example of codominance is the human ABO blood group system. The ABO system is due to the gene that codes for molecules on the surface of red blood cells. In this system, there are three alleles for the same trait viz. A allele, B allele, and O allele. The A and B alleles are codominant and the O allele is recessive to both. The A, B and O allele is expressed as IA, IB, and i respectively. If someone has blood group A, he/she may be homozygous for the A allele i.e., IAIA, or heterozygous for A and O alleles i.e., IAi. If a person has B type of blood, he/she may be homozygous for B allele i.e, IBIB or genotype IBi,
We will see codominance in those individuals who are heterozygous for A and B allele and they will have type AB blood. Both the A and B alleles will be expressed in the phenotype. A person will have type O blood only if he/she is homozygous for O allele i.e., ii.
All possible combinations of ABO alleles could be seen in human populations. However, there is a slight difference in the frequency of different alleles in different ethnic groups.