What term is used for a gene that has more than two alleles?

The term "multiple alleles" is used to describe a gene that has more than two alternative forms, or alleles, in the population. While an individual can carry only two alleles for a particular gene (one inherited from each parent), multiple alleles refer to the different variations that exist for that gene within a broader population.

For example, the ABO blood group system in humans illustrates this concept perfectly, as it is controlled by multiple alleles: A, B, and O. This leads to four possible blood types: A, B, AB, and O. The presence of these multiple alleles increases genetic diversity within a population.

The other terms listed pertain to different aspects of genetics. Co-dominance refers to a situation where two alleles at a gene locus are expressed equally in the phenotype. The term dominant describes an allele that masks the expression of another allele. The word allele specifically refers to an individual variant of a gene. While these terms are related to genetics, they do not capture the concept of having more than two allele forms for a single gene, making "multiple alleles" the correct and most precise choice.