Forward genetics

Forward genetics is the approach of determining the genetic basis responsible for a phenotype. This was initially done by using naturally occurring mutations or inducing mutants with radiation, chemicals, or insertional mutagenesis (e.g. transposable elements). Subsequent breeding takes place, mutant individuals are isolated, and then the gene is mapped. Forward genetics can be thought of as a counter to reverse genetics, which determines the function of a gene by analyzing the phenotypic effects of altered DNA sequences. Mutant phenotypes are often observed long before having any idea which gene is responsible, which can lead to genes being named after their mutant phenotype (e.g. Drosophila rosy gene which is named after the eye colour in mutants).

Often hundreds of thousands of mutations are generated. Chemicals like ethylmethanesulfonate (EMS) cause random point mutations. These types of mutagens can be useful because they are easily applied to any organism but they can be very difficult to map. Mutations can also be generated by insertional mutagenesis. For example, transposable elements containing a marker are mobilized into the genome at random. These transposons are often modified to transpose only once, and once inserted into the genome a selectable marker can be used to identify the mutagenized individuals. Since a known fragment of DNA was inserted this can make mapping and cloning the gene much easier. Other methods such as using radiation to cause deletions and chromosomal rearrangements can be used to generate mutants as well.

Once mutagenized and screened, typically a complementation test is done to ensure that mutant phenotypes arise from the same genes if the mutations are recessive. If the progeny after a cross between two recessive mutants have a normal phenotype, then it can be inferred that the phenotype is determined by more than one gene. Typically, the allele exhibiting the strongest phenotype is further analyzed. A genetic map can then be created using linkage and genetic markers, and then the gene of interest can be cloned and sequenced. If many alleles of the same genes are found, the screen is said to be saturated and it is likely that all of the genes involved producing the phenotype were found.

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