When both alleles for a gene are equally expressed, it's called codominance.

This is best illustrated with human blood types. The ABO blood group system is determined by the *ABO* gene, which has three alleles: A, B, and O.

* A and B alleles are codominant: If you inherit both an A allele and a B allele, you will express both A and B antigens on your red blood cells, resulting in blood type AB.

* O allele is recessive: The O allele doesn't produce any antigens, so it is only expressed if two O alleles are inherited (blood type O).

So, in the case of AB blood type, both the A and B alleles are expressed equally, making it a classic example of codominance.