Genetic screen

A genetic screen (or simply screen) is a procedure or test to identify and select individuals which possess a phenotype of interest. A genetic screen for new genes is often referred to as forward genetics as opposed to reverse genetics that is the term used for identifying mutant alleles in genes that are already known.

Creating a mutant population

Since unusual alleles and phenotypes are rare, geneticists 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. Mutagens such as random DNA insertions by transformation or active transposons can also be used to generate new mutants. These techniques have an advantage since a new allele is tagged with a known molecular (DNA) marker that can facilitate the rapid cloning of the gene without mapping.

Types of screen

A basic screen involves looking for a phenotype of interest in the mutated population. One might screen for obvious phenotypes such a fruit flies with no wings or an Arabidopsis flower with no petals.

More subtle screens is a temperature sensitive screen that involves temperature shifts to enhance the mutant phenotype. A population grown at low temperature would have a normal phenotype, however, the mutation in the particular gene would make it unstable at a higher temperature. 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. An advantage of alleles found in this type of screen is that the mutant phenotype is conditional and can be activated by simply raising the temperature. A null mutation in such a gene may be embryo lethal and such mutants would be missed in a basic screen.

An enhancer/supressor screen is the most sophisticated type of genetic screen. In this case mutagenised population has a weak mutant allele in the biological process of interest. For example, a weak allele of a fly mutant that usually has no wings may have small abnormal wings. In this sensitised background new mutants are discovered that either enhance (small wings to no wings) the phenotype or suppress (small wings to normal wings) the phenotype. Such a screen has two advantages. Firstly, the new genes identified are often involved in the same biological process as the weak allele in the genetic background (in this case wing formation). Secondly, due to genetic redundancy, the mutant genes discovered in such a screen often have no visible phenotype of their own. In a more basic screen they would not be discovered but in the sensitised genetic background a visible phenotype is clear.

Mapping mutants

By the classical genetics approach, a researcher would then locate (map) 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. Classical geneticists would have used phenotypic traits to map the new mutant alleles. With the advent of genomic sequences for model systems such as Dropsophila, Arabidopsis and C. elegans many SNP's have now been identified that can be used as traits for mapping. SNP's are the prefered traits for mapping since they are very frequent, on the order of one difference per 1000 base pairs, between different varieties of organism.