Here's why:

* Alleles: These are different versions of the same gene. For example, the gene for eye color might have an allele for blue eyes and an allele for brown eyes.

* Multiple Alleles: Many genes have more than two alleles. This means there are many different possibilities for the combinations of alleles that an individual can inherit.

* Complexity: Most traits are complex and influenced by multiple genes (polygenic inheritance), as well as environmental factors. This makes the interplay of different alleles even more significant.

Examples:

* Blood Type: The ABO blood group system has three alleles (A, B, and O), resulting in four possible blood types (A, B, AB, and O).

* Hair Color: Hair color is determined by multiple genes, with each gene having several alleles.

* Height: Height is a complex trait influenced by many genes, each with multiple alleles.

Therefore, the concept of multiple alleles is essential for understanding how genes contribute to the variety of traits we see in living organisms.