A Genetic screen is the procedure or step to identify and select the individuals who possess the phenotype of interest. A genetic screen for new genes is often referred as the forward genetics and not the reverse genetics which the term to identify the mutant alleles that are already known. The mutant alleles that are not tagged for the rapid cloning are mapped and cloned by the positional cloning.

How to create 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 Nobel laureates Christiane Nüsslein-Volhard and Eric Wieschaus. A saturation screen is performed to uncover every gene 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.

Type of screening
I would like to describe the three types of screens for you. They are as follows:

1. Basic screen
A basic screen involves the looking for a phenotype of interest in the mutated population. One can really screen for the obvious phenotypes such as the fruit flies with no wing or a flower with no petal.

2. Temperature sensitive screen
More subtle is the temperature sensitive screen that involves the temperature shift to enhance the mutant phenotype. A population growth at low temperature would have a normal phenotype. However the mutation in the particular gene at the higher temperature will definitely make it unstable. You must know that the mutant phenotype is conditional and can be activated by just raising the temperature. A null mutation in such a gene may be lethal for the embryo and such mutation will be missed in the basic screen.

3. An enhancer or the suppressor screens
This is the most sophisticated type of the genetic screen. This type of the screen has the two advantages. First, new genes identified in the screen are often involved in the same biological process as the weak allele in the genetic background, in this case wing formation. Second, due to genetic redundancy, the mutant genes discovered may not have a visible phenotype of their own. In a more basic screen these would not be discovered, however, in the sensitized genetic background a visible phenotype is clear.

I really feel that the biotechnology has given a lot to the field of medicine. We are now on the verge to discover and cure many diseases which were not been able to be cured until now. Genetic screening is really a one small part of this vast field of biotechnology.