What is a gene that has more than two alleles called?

A gene that has more than two alleles is referred to as a multiple allele. This concept arises when a particular gene exists in more than two forms within a population. Each individual typically has two alleles for a given gene, one inherited from each parent; however, the population can exhibit multiple variations of that gene.

For example, in the ABO blood group system, the gene that determines blood type has three common alleles: A, B, and O. Although an individual can only have two of these alleles (for example, one from each parent), the existence of more than two alleles in the population illustrates the concept of multiple alleles.

The other terms provided do not accurately describe this genetic phenomenon. A polygenic trait refers to a characteristic that is controlled by multiple genes. A diploid gene relates to the normal condition of having two copies of each gene (one from each parent) but does not imply multiple alleles. Homologous traits refer to structures that may share a common ancestry but do not pertain to allele variations within a specific gene.