Final answer:
In the ABO blood type system, the IA allele is codominant with the IB allele, allowing both Type A and Type B antigens to be equally expressed in individuals with an AB blood type, showcasing a classic example of codominance and multiple alleles. So, the correct answer is option b.
Step-by-step explanation:
In the ABO blood type system, the IA allele is codominant with the IB allele. Codominance is a form of inheritance where both alleles in a heterozygous individual contribute to the phenotype. In the case of ABO blood types, individuals with one IA allele and one IB allele express both Type A and Type B antigens on their red blood cells, resulting in the AB blood type. This is distinct from a situation of complete dominance, where only one allele's trait would be expressed, and different from incomplete dominance, where an intermediate phenotype would be observed.
Codominance in the ABO blood group system is also accompanied by the presence of multiple alleles within the population: IA, IB, and i. While IA and IB are codominant with each other, they are both dominant over the i allele which results in Type O blood when an individual has two copies of the i allele.
So, the correct answer is option b.