How does codominance work in the human MN blood group system?

In the human MN blood group system, codominance is a genetic phenomenon where two different alleles—designated as M and N—are expressed to an equal degree, allowing the traits associated with each to be displayed simultaneously.12

The system functions as follows:

• Homozygous Genotypes: Individuals who are homozygous for the M allele (MM) possess only the M antigen on the surface of their red blood cells. Conversely, those homozygous for the N allele (NN) possess only the N antigen.12
• Heterozygous Genotypes: Individuals who are heterozygous (MN) carry both alleles. Because the alleles are codominant, these individuals express both the M and N antigens on their red blood cells simultaneously.12

These antigens are encoded by the GYPA gene, which produces glycoprotein structures on the cell membrane. While the presence of both antigens defines the codominant phenotype, the specific amount of antigen expressed can vary depending on the individual's genetic constitution.345