Final answer:
In human ABO blood types, the alleles for A (IA) and B (IB) are codominant. This means that a person with both alleles will have type AB blood, expressing both antigens. Codominance is a pattern where both alleles are equally and distinctly expressed, unlike incomplete dominance.
Step-by-step explanation:
ABO Blood Type and Codominance
In human ABO blood types, the alleles for A (IA) and B (IB) are said to be codominant because both alleles are expressed equally in the phenotype. This means that a person with one A allele and one B allele will have AB blood type, exhibiting both A and B antigens on their red blood cells. For blood type A, possible genotypes are either homozygous (IAIA) or heterozygous (IAi), while for blood type B, the genotypes can be either homozygous (IBIB) or heterozygous (IBi). People with two recessive alleles (ii) have blood type O, which is neither A nor B, since the i allele does not produce the corresponding antigen.
The fact that neither the A nor the B allele is recessive when paired with each other is what characterizes codominance in this system. This pattern of inheritance is distinct from incomplete dominance, where a blended phenotype is expressed by the heterozygote. Thus, in the context of ABO blood groups, the inheritance pattern is an example of codominance with multiple alleles.