Alleles are seen as an alternative form of a gene, and they are usually responsible for differences in the phenotypic expression of a particular trait e.g., brown eyes versus green eyes. A gene in which at least two alleles exist is referred to as polymorphic. In a situation in which a particular gene may exist in three or more allelic forms are known as multiple allele conditions. It is so essential to note that while multiple alleles occur and are maintained within a population, anybody who possesses only two such alleles at equivalent loci on homologous chromosomes. A good example of multiple alleles is the ABO blood-type system in humans. In this situation, there are three alleles circulating in the population. ( IA allele codes in A molecules on the red blood cells, I allele codes for no molecules on the red blood cells and IB allele codes for B molecules on the surface of red blood cells). In this, the IA and IB alleles are said to be codominant with each other and both are dominant over the I allele. Though there are three alleles present in a population, each person only gets two of the alleles from their parents. This produces the genotypes and phenotypes shown in the diagram below. It is important to notice that instead of three genotypes, there are six different genotypes when there are three alleles. The rate of possible phenotypes depends on the dominance relationships between the three alleles.
The Inheritance of the ABO blood system in humans is shown.
In the concept of Mendelian inheritance, a given locus of the chromosome was occupied by 2 kinds of genes, i.e., a normal gene for round seed shape and other its mutant recessive gene for wrinkled seed shape. But it can be possible that the normal gene may show still multiple mutations in pea besides the one for wrinkledness. In this case, the locus will be occupied by normal allele and its two or more mutant genes. Therefore, three or more kinds of genes occupying the same locus in an individual chromosome are referred to as multiple alleles.
Multiple Alleles. A different form of wings drosophila
In Drosophila wings are usually long. Which occurred two mutations at the same locus in different flies, one causing vestigial wings and other mutation causing antlered wings. Both alleles are of the same normal gene and also of each other and are recessive to the normal gene.