Alleles are

Genes which code for a pair of contrasting traits are known as alleles, i.e., they are slightly different forms of the same gene.Each gene is a sequence of nucleotides at a specific place, or locus, along a particular chromosome. The DNA at that locus, however, can vary slightly in its nucleotide sequence. This variation in information content can affect the function of the encoded protein and thus the phenotype of the organism. Example: The purple-flower allele and the white-flower allele are two DNA sequence variations possible at the flower-color locus on one of a pea plant’s chromosomes, one that allows synthesis of purple pigment and one that does not.