Multiple Alleles

c. Multiple alleles :

Multiple allelism describes a gene that exists as more than two alternative forms (alleles) circulating within a population, all occupying the same locus on homologous chromosomes, even though any single diploid individual can only ever carry two of these alleles at once. Such series arise because a wild-type gene can mutate repeatedly, at different sites or in different ways, producing a graded set of alternative alleles rather than a single mutant form.

The chapter's example is wing size in Drosophila, where a whole series of alleles at one locus produces phenotypes ranging from fully normal wings, through progressively more reduced nicked, notched and strap wings, down to vestigial wings that are barely more than stumps — with the wild-type allele dominant over every mutant allele in the series, and the different mutant alleles able to show dominant-recessive, co-dominant or incompletely dominant relationships among themselves.