What are multiallelic genes?
More than two alternative forms (alleles)of a gene in a population occupying the same locus on a chromosome or its homologue is known as multiple alleles.

Multiple alleles arise due to mutations of gene.A gene can mutate several times by producing a series of alternative expressions.Different alleles in a series show dominant-recessive relation or may show co-dominance or incomplete dominance among themselves.

In Drosophila,a large number of multiple alleles are known.One of them is the series of wing abnormality ranging in size from normal wings to no wings.The normal wing is wild type.The extreme expression with no wings I.e is just stumps is due to one allele 'vg',in homozygous condition .

For reference.
http://eagri.tnau.ac.in/eagri50/GBPR111/lec04.pdf