Alleles are polymorphs that differ in their nucleotide sequence resulting in contrasting phenotypic expression. Alleles are the alternative forms of a same gene, e.g. genes for height have two alleles, one for dwarfness (t) and one for tallness (T). Significance of alleles are:
(i) A character may have two or more contrasting phenotypic expression, thus resulting in variation in a population.
(ii) These are used in the studies of inheritance and in understanding their behaviour.