Genetic screens are analytical tools for studying the correlation between the genotype and the phenotype of an organism. They are broadly classified into forward genetic screens and reverse genetic screens.

Forward screens involve identifying the genes responsible for an observed characteristic, such as petal color, thus using a known phenotype to study an unknown genotype.

These screens usually involve the genome of an organism being randomly mutated to induce a change in phenotype. Next, the mutant organisms are bred until genes are homozygous, to ensure expression of recessive phenotypes. Organisms are then screened for traits of interest.

For example, to identify genes responsible for morphological defects in zebrafish embryos, sperm cells of adult males are mutated with a chemical, ethylnitrosourea, and then crossed with a wild type female.

The resulting F1 males are crossed again with a wild type female and dominant mutations observed in the F2 progeny. F2 individuals are then inbred to identify recessive gene mutants in F3 individuals.   

Genes responsible for the altered phenotype can then be determined by different methods. In one technique, genomes of multiple mutant and wild type organisms are sequenced.  A region that is similar in all mutants, but not the wild types, helps in locating the mutated gene.

Additionally, forward genetic screens can be used to identify mutations that enhance or suppress the severity of a phenotype in organisms. These are known as modifier screens.

In contrast, reverse genetic screens go from genotype to phenotype and examine the phenotype resulting from mutating a specific gene with an unknown function.

For example, consider the genes for molecular chaperones in Drosophila.  Molecular chaperones are proteins essential for the folding of other proteins in different parts of the organism. Targeted knockdown of different chaperone genes in Drosophila eyes can result in observable morphological defects, thereby helping to identify which of these chaperones are essential for eye development.

Alternatively, expression screening is another reverse genetics method. Here, a gene with an unknown function is expressed in a different organism, and changes in the phenotype are observed in the host to determine gene function.