forward genetics
Forward genetics works from the visible outward, asking “something is different about this organism — which gene caused it?” You start with an interesting trait or defect, often produced by random mutation, and then track down the gene responsible. It is detective work: you have the symptom and must find the culprit.
In practice, researchers treat many organisms with a mutagen to create random mutations, then screen the offspring for individuals showing a phenotype of interest, such as a developmental abnormality. By breeding these mutants and mapping where the responsible mutation lies on the chromosomes, they narrow down and finally identify the affected gene.
The great strength of forward genetics is that it makes no assumptions about which genes matter: it lets the organism reveal them by their effects, so it can uncover completely unexpected players in a process. Its cost is labor — large screens and painstaking mapping — and it works best in organisms that breed fast and cheaply.
A researcher notices a strain of flies whose wings curl, breeds them to confirm the trait is inherited, maps the mutation to a chromosome region, and finally pinpoints the single gene responsible.
From a striking phenotype back to the gene behind it.
Forward genetics moves from phenotype to gene; reverse genetics goes the other way, from a chosen gene to its phenotype. Modern sequencing has made the gene-finding step of forward genetics far faster than it once was.