Gene duplication (or chromosomal duplication or gene amplification) is any duplication of a region of DNA that contains a gene; it may occur as an error in homologous recombination, a retrotransposition event, or as duplication of an entire chromosome. The second copy (duplicate) of the gene is often free from selective pressure — that is, mutations of it have no deleterious effects to its host organism. Thus, it accumulates mutations at a more rapid rate than a functional single-copy gene.

A duplication is the opposite of a deletion. Duplications arise from an unbalanced recombination that occurs during meiosis between misaligned homologous chromosomes. The chance of this happening is a function of the degree of sharing of repetitive elements between two chromosomes. The product of this recombination is a duplication at the site of the exchange and a reciprocal deletion

Because of the relative freedom a duplicate gene has from natural selection, it is thought to be a key player in evolution. For instance, major genome duplication events that increase the ploidy of an organism are not uncommon; plants are prolific genome duplicators - wheat, for example, is hexaploid (carrying six copies of its genome).

The freedom from consequences in gene duplicates allows for the mutation of novel genes that could potentially increase the fitness of the organism or code for a new function. An example of this is the apparent mutation of a duplicated digestive gene in a family of ice fish into an antifreeze gene.

The two genes that exist after a gene duplication event are called paralogs and usually code for proteins with a similar function and/or structure. By contrast, orthologous genes are ones which code for proteins with similar functions but exist in different species, and are created from a speciation event. It is important (but often difficult) to differentiate between paralogs and orthologs in biological research. Experiments on human gene function can often be carried out on other species if a homolog to a human gene can be found in the genome of that species, but only if the homolog is orthologous. If they are paralogs and resulted from a gene duplication event, their functions are likely to be too different.

Gene duplication doesn't necessarily constitute a lasting change in a species' genome. In fact, such changes often don't last past the initial host organism. From the perspective of molecular genetics, amplification is one of many ways in which a gene can be overexpressed. Genetic amplification can occur artificially, as with the use of the polymerase chain reaction technique, or it can occur naturally, as described above. If it's a natural duplication, it may take place in a somatic cell rather than a germline cell (which would be necessary for a lasting evolutionary change).

Also, in either event, duplications can be and often are marginally or severely detrimental. For instance, duplications of oncogenes are a common cause of many types of cancer. In such cases the genetic duplication occurs in a somatic cell and affects only the genome of the cancer cells themselves, not the entire organism, much less any subsequent offspring. Genomic microarrays can be involved in the detection of duplications.