In the ABO blood group system, which alleles are considered codominant?

In the ABO blood group system, the alleles IA and IB are considered codominant because when both are present in an individual, they express both phenotypes simultaneously, resulting in the AB blood type. Codominance occurs when neither allele is dominant or recessive, allowing both to be fully expressed in the presence of each other.

This means that a person with one IA allele (which corresponds to type A blood) and one IB allele (which corresponds to type B blood) will have type AB blood, demonstrating the presence of both antigens on the surface of red blood cells.

In contrast, the other alleles like i are recessive to both IA and IB. Hence, when an individual has an IA allele and an i allele, the dominant IA will express type A blood. This codominance observed between IA and IB is vital for understanding blood types and predicting compatibility in blood transfusions.