More than two alternative forms (alleles) of a gene in a population occupying the same locus on a chromosomes or its homologue are known as multiple alleles. Multiple alleles arise by mutations of the wild type of gene. A gene can mutate several times producing a series of alternative expressions. Different alleles in a series show dominant - recessive relation or may show co-dominance or incomplete dominance among themselves. (An individual is with any two alleles of a gene, one on each chromosome of the homologous pair). Wild type is dominant over all other mutant alleles. Multiple alleles do not undergo crossing over.
In Drosophila, a large number of multiple alleles are known. One of them is the series of wing abnormality ranging in size form normal wings to no wings. The normal wings is wild type and is denoted by Vg +. The extreme expression with no wings i.e. just stumps, is due to one allele vg, in homozygous condition. Following table shows the complete series.
Another good examples of multiple alleles is ABO blood groups in human beings.
The gene I controls the ABO blood groups. It is three allele; IA. ,IB and i.
The allele IA and IB produce a slightly different from of the sugar and allele i, does not produce any sugar. Since humans are diploid organisms, each person possesses any two of the three I gene alleles. IA and IB are co-dominant over i. There are six different genotypes and only four different phenotypes, i.e. blood group are follows: