Genetic screen
Genetic screens can provide important information on gene function as well as the molecular events that underlie a biological process or pathway.
For instance, the famous screen by Christiane Nüsslein-Volhard and Eric Wieschaus mutagenized fruit flies and then set out to find the genes causing the observed mutant phenotypes.
Christiane Nüsslein-Volhard and Eric Wieschaus were the first individuals to perform this type of screening procedure in animals.
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 famous temperature-sensitive screen was carried out independently by Lee Hartwell and Paul Nurse to identify mutants defective in the cell cycle in S. cerevisiae and S. pombe, respectively.
Similar to classical genetic screens in the past, large-scale RNAi surveys success depends on a careful development of phenotypic assays and their interpretation.
It can also be used to study functional consequences of mutations in vivo by enabling direct genome editing in somatic cells.
With the advent of genomic sequences for model systems such as Drosophila melanogaster, Arabidopsis thaliana and C. elegans many single nucleotide polymorphisms (SNPs) have now been identified that can be used as traits for mapping.
In fact, the Heidelberg screen, allowing mass testing of mutants and developed in 1980 by Nüsslein-Volhard and Wieschaus, cleared the way for future scientists in this field.
These techniques have the advantage of tagging the new alleles with a known molecular (DNA) marker that can facilitate the rapid identification of the gene.
Positional cloning typically involves the isolation of partially overlapping DNA segments from genomic libraries to progress along the chromosome toward a specific gene.
Tests used for this purpose include cross-species hybridization, identification of unmethylated CpG islands, exon trapping, direct cDNA selection, computer analysis of DNA sequence, mutation screening in affected individuals, and tests of gene expression.
For each new DNA clone a polymorphism is identified and tested in the mapping population for its recombination frequency compared to the mutant phenotype.
Modern positional cloning can more directly extract information from genomic sequencing projects and existing data by analyzing the genes in the candidate region.
