Genes may often exist in more than one form, and these forms are termed alleles of the gene. Accordingly, an allele is one of at least two alternative forms of a particular gene. Alleles provide the genetic instructions for products that, although similar in type, are visibly different (phenotypically different). The term allele is derived the Greek term alleon used to describe a difference in morphology or form. At the genetic level, alleles contain differing base sequences in their nucleic acid (e.g., DNA). As a form of a gene, an allele carries the instructions for a particular variation of the gene's protein product. In immunogenetics, an allotype is the protein product of an allele that is detectable as an antigen (any molecule that induces the formation of an antibody) by another organism of the same species.

Although underlying genetic molecular complexities sometimes blur...