An allele (pronounced al-eel or al-e-ul) is any one of a number of viable DNA codings that occupies a given locus (position) on a chromosome. Usually alleles are DNA (deoxyribonucleic acid) sequences that code for a gene, but sometimes the term is used to refer to a non-gene sequence. An individual's genotype for that gene is the set of alleles it happens to possess. In a diploid organism, one that has two copies of each chromosome, two alleles make up the individual's genotype. The word came from Greek αλληλος = "each other".
An example is the gene for blossom colour in many species of flower—a single gene controls the colour of the petals, but there may be several different versions (or alleles) of the gene. One version might result in red petals, while another might result in white petals. The resulting colour of an individual flower will depend on which two alleles it possesses for the gene and how the two interact.