What is described by the term 'multiple alleles'?

The term 'multiple alleles' refers to the presence of three or more alternative forms of a gene that can exist at a single genetic locus. This can lead to a variety of phenotypes within a population, as each individual may carry combinations of these alleles.

For instance, classic examples of multiple alleles include the ABO blood group system in humans, where there are three alleles—A, B, and O—each contributing to the blood type of an individual. While an individual has only two alleles (one from each parent) for any given gene, the existence of multiple alleles in the population creates a pool of genetic variation that can express different traits.

The other options do not accurately convey the concept of multiple alleles. Having two alleles for a gene does not encompass the idea of multiple alleles, which explicitly requires at least three variations. Single allele dominant traits and alleles that have no effect on phenotype also do not represent the broader concept of multiple alleles, as they either refer to specific conditions or lack the diversity required to define multiple allelic forms.