In immunology, the "immunoglobulin (AKA antibody) isotype" refers to the slight phenotypic variations within the immunoglobulin gene family that encode for variant immunoglobulin heavy chains and immunoglobulin light chains. To be an isotype of a particular gene family means to belong to a set of genes that encode functionally similar proteins. Usually, isotypes that belong to a particular gene family are duplicate copies of the same gene that arose because of a mistake during DNA replication. As the species continues to evolve, those duplicated genes are passed on from generation to generation until every member of the species has a copy. Each of those genes have since undergone mutations of their own, such that they diverge slightly in structure and function, giving rise to the different isotypes known today.