Answer: Alleles are the pairs of genes occupying a specific spot called locus on a chromosome. Typically, there are only two alleles for a gene in a diploid organism. When there is a gene existing in more than two allelic forms, this condition is referred to as multiple allelism. Allelism refers to any of the several forms of a gene. These genetic variations arise usually through mutation and therefore are responsible for hereditary variations.
In Drosophila, eye color is controlled by 15 alleles, wild type or normal allele is 'W' which produces red eye color which is dominant over all other alleles. Allele 'w' codes for white eye color which is recessive to all other alleles.