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A polymorphic gene locus is defined as a locus that has two or more alleles present in a population, which can result in a variety of traits or phenotypes among individuals. This variation indicates that there are multiple forms of the gene existing within the population, allowing for genetic diversity.

In contrast, a monomorphic gene locus has only one allele for a particular gene within a population, leading to less variation in traits. Homozygous and heterozygous loci refer to the specific genetic combinations in individuals: homozygous means that both alleles at a locus are the same, while heterozygous means that the alleles are different. These terms do not address the presence of multiple allele options within the population, which is the key characteristic of a polymorphic locus.