Mendel's concept of inheritance envisaged alternative forms of same gene or factor. This would mean that each gene had two alternative forms or allelomorphs, one being dominant or partially dominant and the other recessive; one being wild form and the other mutant. If the mutant form developed from wild form due to mutation, there is no reason, why we should not expect that the wild form can mutate in more than one ways. The mutant form can also mutate once again to give rise to another mutant form. Therefore, a gene can have more than two allelomorphs. These allelomorphs make a series ef multiple alleles. Relationship between different alleles may or may not be a simple dominant-recessive relationship, but they are usually characterized by their location on the same locus, and therefore, by lack of crossing over between them. However, discovery of pseudoalleles has rendered this definition out of date, that multiple alleles, can occupy different sites within a gene and that crossing over between them can take place.
It is also possible that several alleles may exhibit themselves within the same phenotypic range, whether wild or mutant. Such allelesexpressing themselves within the same phenotypic range are called isoalleles.
If the phenotype is wild, these are normal
or wild isoalleles
and if the phenotype is mutant, they will be called mutant isoalleles.
Such examples are many and one of them is noticed in eye colour in Drosophila,
where several alleles exhibit red eye colour (W+s, W+c, W+g).
The phenomenon of multiple allelism will be illustrated in this section with the help of a number of examples, both from animals and plants.