In the universal ABO blood grouping system, there are four phenotypes—A, B, AB, and O—which are determined by three different alleles. The presence of three alleles in the ABO blood grouping system is an example of .

The correct answer is - autosomal codominant inheritance (due to multiple alleles).

Explanation:

Autosomal codominant inheritance or codominance is a pattern of inheritance where the alleles present of an autosomal gene express their character in the phenotype. In other words, in this type of pattern, no allele would be masked by other alleles and both will be expressed in the phenotype.

The human ABO blood group is an example of this pattern as there are three alleles A, B, and O. In the heterozygous case of AB blood group both A, and B alleles expressed together however, O is masked by both alleles A, and O.