What is an example of a trait determined by multiple alleles?

Blood types serve as a prime example of a trait determined by multiple alleles. In humans, the ABO blood group system is influenced by three alleles: A, B, and O. The presence of these alleles results in four possible blood types: A, B, AB, and O. This system exemplifies multiple allelism because more than two distinct alleles influence the expression of one trait—blood type—in the population.

In contrast, while traits like eye color, height, and skin color may also be influenced by multiple genes (polygenic inheritance), they do not involve multiple alleles of a single gene in the same way blood types do. Therefore, blood types specifically illustrate the concept of multiple alleles as they illustrate how the variation in one gene can produce several phenotypes.