Reverse genetics is an approach to genetics research which takes an unknown genetic sequence and attempts to figure out the phenotype with which it is associated. This is in contrast with classical genetics, in which researchers take a known phenotype, such as stripes on a cat, and try to determine which gene or genes cause the phenotype to appear. These two different approaches are both in widespread use, and contribute to a greater overall understanding of the genome of many organisms.
As gene sequencing became faster and easier, researchers began to be faced with a wealth of material they knew nothing about. They could detect clear genetic sequences, but they had no idea what these sequences did or didn't do. As a result, reverse genetics began to be used to explore these unknown areas of the genome to learn more about their function. Given the length of the genome in some organisms, there will be ample material to explore for many generations of researchers.
In reverse genetics, a researcher takes an unknown genetic sequence and manipulates it to see how it changes the phenotype. Researchers can introduce mutations, deletions, additions, or other changes to the sequence which should change the way in which it expresses. They can also turn genetic sequences off and on. By observing changes caused by the manipulation, a researcher can draw conclusions about the function of the sequence in the body.
One could think of reverse genetics as a sort of black box area of research. In this case, the genetic sequence is inside the black box, with the function being hidden from the researcher. The researcher sticks a metaphorical hand into the box to manipulate the contents, and then sees how the manipulation changes the output of the box. Using reverse genetics, people can figure out the function of specific sequences and areas of the genome.
One area in which reverse genetics is used is in the development of vaccines, with researchers taking new viruses and exploring their genetic codes to find the right segments to introduce to a vaccine for the purpose of protecting people. Reverse genetics can sometimes turn up remarkable surprises, because researchers don't necessarily know what their manipulations will accomplish. They may discover links to genetic conditions, find out that the expression of a trait is more complicated than previously believed, or find new approaches to medical treatment through a deeper understanding of the function of the genome.