Final answer:
The ABO blood group system is an example of codominance in the phenotype.
Step-by-step explanation:
The ABO blood group system is an example of codominance in the phenotype.
Codominance occurs when both alleles in a heterozygous individual are expressed equally without one allele being dominant over the other.
In the case of ABO blood groups, individuals with the IA and IB alleles will have type AB blood, in which both the A and B antigens are expressed equally.