Codominance occurs when both alleles contribute expression to the phenotype;
an example is human blood types.
4 phenotypes from the 3 alleles are possible:
- Type A is either IAIA or IAi.
- Type B is either IBIB or IBi.
- Type AB is IAIB.
- Type O is ii.
The IA and IB alleles are both
dominant over i.
The IA and IB alleles are codominant.