An allele is one of usually two or more forms of genes (generally a group of genes). An allele is an alternative form of a gene that is located at a specific position on a specific chromosome. They determine distinct traits that can be passed on from parents to offspring. The process by which alleles are transmitted was discovered by Gregor Mendel and formulated in what is known as Mendel's law of segregation.
Organisms have two alleles for each trait. When the alleles of a pair are heterozygous, one is dominant and the other is recessive. The dominant allele is expressed and the recessive allele is masked.
Example - The gene for seed shape in pea plants exists in two forms, one form or allele for round seed shape (R) and the other for wrinkled seed shape (r). Round seed shape (R) is dominant and wrinkled seed shape (r) is recessive. So Round is expressed as (RR) or (Rr) and Wrinkled as (rr).