Multiple Alleles

So far, it has been observed that a given phenotypic trait of an individual depends on a single pair of genes, each of which occupies a specific position called the gene locus, on a homologous chromosome. Moreover, a particular gene has been found to occur in two alternative forms. For example, a gene (L) for length of Drosophila wings may occur in two alternative forms: a gene (L+) for normal development of wings and a gene (Lvg) for vestigial wings. Because, most flies have normally developed wings, so, it can easily De concluded that gene L + is the original form of gene from which the other form of gene (Lvg) might have originated by certain mutational event at sometime in past. The gene L+ for normal development of wings is called the normal or wild type allele of the gene L and usually symbolized as L+, while the mutated gene Lvg for vestigial wings is called Lvg reduced type or mutant allele of gene L.


A fly with normal wings, thus has two wild type alleles (L+L+) and the vestigial wings fly has two mutant alleles (Lvg Lvg). Both of these allelic forms (L + and Lvg) of gene L occur at corresponding positions on genetically identical (homologous) chromosomes of same or different individual.

But, now there are ample evidences that a gene for a particular character, besides occurring in two alternative forms or alleles may occur in several alternative forms or alleles. All the variants or alleles of a given gene are supposed to be originated by mutation of a single wild type gene. Out of several allelic forms of a gene, a given locus may bear anyone allele, so that, a diploid individual possesses any two alleles of the allelic series. When any of the three or more allelic forms of a gene occupy the same locus in a given pair, of homologous chromosomes they are said to constitute a series of multiple alleles. In other words, all the mutant forms of a single wild type gene constitute a series of multiple alleles.

Characters of Multiple Alleles -
The most important and distinguishing features of multiple alleles are summarized below:
1. Multiple alleles of a series always occupy the same locus in the chromosome.
2. Because, all the alleles of multiple series occupy the same locus in chromosome, therefore, no crossing-over occurs within the alleles of a same multiple allele series.
3. Multiple alleles always influence the same character.
4. The wild type allele is nearly always dominant, while the other mutant alleles in the series may show dominance or there may be an intermediate phenotypic effect.
5. When any two of the mutant multiple alleles are crossed, the phenotype is mutant type and not the wild type.

Symbolism for Multiple Alleles-
The dominance hierarchy is defined at the beginning of each problem involving multiple alleles. A capital letter is commonly used to designate the allele which is dominant to all other alleles in the series. The corresponding lowercase letter designates the allele which is recessive to all others in the series. Other alleles which are intermediate in their degree of dominance between these two extremes, are usually assigned the lower case letter with some suitable super script.