Reverse genetics is an approach to discovering the function of a gene that proceeds oppositely to how such discoveries typically unfold in classical genetics.
Classical and reverse genetics are alike in that, by either approach, investigators typically must deduce the function of a normal gene from the effects that follow from damaging or changing it. Otherwise, the two approaches contrast.
By the classical approach, geneticists first look for rare individuals with unusual traits or phenotypes, and then they trace these traits to an underlying faulty allele or gene. Locating the gene on its chromosome is the end point of an investigation.
With the readily performed modern techniques of DNA sequencing and as a result of the sequencing of many whole genomes, many genetic sequences are discovered in advance of any other information about them. To learn the influence a sequence has on phenotype, or to discover its biological function, researchers may engineer a change or disruption in it--by site-directed mutagenesis, for example, or by gene knockout--and only afterwards look for the effect of such alterations in the whole organism. So phenotype, rather than the starting point, is in reverse genetics the end point.
