When there are more than two alleles present in a population, it is called polymorphism.

Here's a breakdown:

* Allele: A variant form of a gene.

* Polymorphism: The existence of multiple alleles for a gene within a population.

Examples:

* Blood type: The ABO blood group system is a classic example. There are three alleles (A, B, and O), leading to multiple possible blood types (A, B, AB, and O).

* Eye color: Multiple genes contribute to eye color, but even within a single gene (like the OCA2 gene), there can be multiple alleles leading to various shades of brown, blue, green, and hazel.

Note: The term "polymorphism" is often used interchangeably with "genetic variation". However, strictly speaking, polymorphism refers to the existence of multiple alleles for a specific gene, while genetic variation encompasses a broader range of genetic differences within a population.