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Forward genetics is the approach of determining the genetic basis responsible for a phenotype. This was initially done by generating mutants by using 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).

General technique
Often hundreds of thousands of mutagens are generated. Chemicals like ethylmethanesulfonate (EMS) cause random point mutations. These types of mutagens can be useful because they’re easily applied to any organism but they can be very difficult to map. Mutagens can also be mutagenized 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.

Human diseases
Before 1980 very few human genes had been identified as disease loci until advances in DNA technology gave rise to positional cloning and reverse genetics. Discovering disease loci using old forward genetic techniques was a very long and difficult process and much of the work went into mapping and cloning the gene through association studies and chromosome walking. Cystic fibrosis however demonstrates how the process of forward genetics can elucidate a human genetic disorder. Genetic-linkage studies were able to map the disease loci in cystic fibrosis to chromosome 7 by using protein markers. Afterward, chromosome walking and jumping techniques were used to identify the gene and sequence it. Forward genetics can work for single-gene-single phenotype situations but in more complicated diseases like cancer, reverse genetics is often used instead.

Classical forward genetics
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. Eventually the hope is that such screens would reach a large enough scale that most or all newly generated mutations would represent a second hit of a locus, essentially saturating the genome with mutations. This type of saturation mutagenesis within classical experiments was used to define sets of genes that were a bare minimum for the appearance of specific phenotypes. However, such initial screens were either incomplete as they were missing redundant loci and epigenetic effects, and such screens were difficult to undertake for certain phenotypes that lack directly measurable phenotypes. Additionally a classical genetics approach takes significantly longer.

Original introduction paragraph
Below is the original introduction paragraph. I decided to completely redo it because it was not cited at all and I believe it was plagiarized from at the very least this source: http://www.innovateus.net/health/what-field-reverse-genetics

"Forward genetics encompasses several means of identifying the genotype that is responsible for a phenotype. Initially, this was done by generating random mutations in an organism, often with radiation or insertional mutagenesis (e.g. through the Sleeping Beauty system), and after subsequent breeding, isolating individuals with an aberrant phenotype. Today there are many means of altering the genotype to observe the phenotype, several of which are useful in the study of functional genomics. Forward genetics can be thought of as a counter to reverse genetics, which seeks to alter genes in order to illuminate their multiple phenotypes. Traditionally, forward genetics involves the use of mutagens (e.g. EMS) and the underlying phenotype would be observed by genetic mapping. Reverse genetics however, due to the advent of advances in sequencing is employed to seek out what sequences underlies the phenotype."