One of the alternative forms of a gene or any other particular site (locus) on a chromosome. In a diploid cell there are usually two alleles of any one gene (one from each parent), which occupy the same locus on homologous chromosomes. These alleles may be the same, or one allele may be dominant to the other (known as the recessive), i.e. it determines which aspects of a particular characteristic the organism will display. Alternatively, neither allele may be completely dominant (see incomplete dominance), or both alleles may be expressed fully (see codominance). Within a population there may be numerous alleles of a gene; each has a unique nucleotide sequence.