A gene is a heritable unit of DNA, found at a specific locus (fixed position) on a chromosome.

A gene typically codes for either a protein or a functional RNA (known as the gene product). All genes typically possess regulatory regions, such as promoter sequences, coding regions which may be transcribed into RNA, and other functional sequences. Genes usually have variations in their DNA sequence leading to slightly different 'versions' of the gene, with the same innate function but slightly different phenotypic effects. Different 'versions' of a gene are called alleles. All the individuals of a species will largely have the same genes, but different alleles of those genes.

Genes may be organised into clusters called operons, that are transcribed together from a single promoter sequence, or alternatively clusters called regulons, which are groups of genes regulated by the same regulatory protein. This is more common in prokaryotes than in eukaryotes, where genes tend to be independently regulated and transcribed.