A genetic screen (or simply screen) is a procedure or test to identify and select individuals which possess a phenotype of interest. A screen for temperature sensitivity in fruit flies, for example, might involve raising the temperature in the cage until some flies faint, then opening a portal to let the others escape. Individuals selected in a screen are liable to carry an unusual version of a gene involved in the phenotype of interest. By the classical genetics approach, a researcher would then locate the gene on its chromosome by crossbreeding with individuals that carry other unusual traits and collecting statistics on how frequently the two traits are inherited together. As a result, screening can enable a geneticist to connect a trait to its genetic underpinnings.
Because unusual alleles and phenotypes are rare, geneticists often expose the individuals that are to be screened to a mutagen, such as a chemical or radiation, which generates mutations in their chromosomes. The use of mutagens enables "saturation screens," one of the first of which was performed by Christiane Nüsslein-Volhard and Eric Wieschaus . A saturation screen is performed to uncover every genes that is involved in a particular phenotype in a given species. This is done by screening and mapping genes until no new genes are found.