Final answer:
Codominance in ABO blood types occurs when both A and B antigens are expressed equally, as in blood type AB with the genotype IAIB. IA and IB alleles are codominant, while both are dominant over the 'i' allele leading to blood type O.
Step-by-step explanation:
Codominance in the ABO blood type system is exemplified by the AB blood type, which has the genotype IAIB. This is because individuals with the AB blood type express both the A and B antigens on their red blood cells simultaneously, and neither antigen is dominant over the other. Therefore, option 3 (AB: IAIB) is the correct answer, as it represents the codominant relationship.
While individuals with blood type A can have genotypes IAIA or IAi, and those with blood type B can have genotypes IBIB or IBi, these are not examples of codominance; instead, they represent situations where one allele is dominant over the other (i). The alleles IA and IB are codominant with each other, while both are dominant in relation to the recessive allele 'i'. Thus, people with two recessive alleles (ii) have blood type O, which is not codominant but rather, reveals the absence of dominant alleles.