How are alleles of particular gene differ from each other? Explain its significance

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.