13.3: Forward and reverse genetics

Originally, genetic analyses were carried out through what is now known as forward genetics. This involves the generation of mutations by chance and then identifying those individuals that carry mutations that disrupt a particular process or structure of interest. As an example, consider eye shape or color in the fruit fly Drosophila melanogaster (→); these are traits that are experimentally accessible because a Drosophila embryo can develop into a fertile adult without eyes. It is therefore possible to identify mutant alleles that alter the eye but allow other aspects of embryonic development to occur (more or less) normally, at least in the context of the laboratory. When we think about a particular trait or behavior, a specific phenotype, we want to know how many different genes are involve in producing or influencing that phenotype. On the other hand, if the product of the mutated gene plays multiple roles in the developing organism, perhaps in processes distinct from those involved in the formation of the eye, the embryo may die before eyes form and no mutations in that gene will be recovered, even though the gene’s product plays a key role in eye development or pigmentation. It is for this reason that forward genetic screens for mutations that influence a particular process are never "complete", that is, they cannot identify every gene/gene product involved in a process.

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