What inheritance pattern is observed in the ABO blood group system?

The inheritance pattern observed in the ABO blood group system is characterized by both codominance and multiple alleles. In the ABO system, there are three main alleles: A, B, and O. The presence of these multiple alleles allows for various combinations, resulting in four possible blood types: A, B, AB, and O.

Codominance is also a key feature; when both the A and B alleles are present in an individual (as in blood type AB), they are both fully expressed, which means that an individual with this genotype will have both A and B antigens on their red blood cells. This phenomenon differentiates ABO blood types from simple dominance patterns, where only one allele would dominate over another.

Thus, the combination of multiple alleles contributing to different phenotypes and the codominance seen when both A and B alleles are present makes the option that encompasses both of these concepts the most accurate description of inheritance in the ABO blood group system.