The allele that exist in more than two different forms are called:

Explanation

In polygenic inheritance, each allele has a cumulative or additive effect on the phenotype thus generating continuous variation, e.g. skin pigmentation, height, etc. There is no such thing as a polygenic allele. Therefore, A is incorrect. Multigenic inheritance is another word for polygenic inheritance so the same applies here, there is no such thing as a multigenic allele. Therefore, B is incorrect. Multiple alleles of a gene occupy the same locus on the chromosome and are the alternative forms of the same gene so they influence the same trait. They usually exist in more than two forms, for example, the gene-determining blood groups. Therefore, the answer is C. Allelic heterogeneity is the phenomenon in which different mutations at the same locus lead to the same or very similar phenotypes. These allelic variations can arise as a result of natural selection processes, as a result of exogenous mutagens, genetic drift, or genetic migration. Therefore, D is incorrect.