Reverse Genetics

One of the major tasks of genetics is to associate phenotypes with the corresponding mutated genes. In contrast, reverse genetics starts with genes, and aims at mutating them to study the resulting phenotype. The two disciplines are separated by many decades, and a change of context.

Initially, genetics was a rather unique field in Biology, for it didn’t need much help from other fields. Geneticists identified new mutant phenotypes, and set various relationships between these mutants (complementation, epistasis, etc…). They could in this way build consistent theoretical systems where genes could be organized into biological pathways with a hierarchy of interactions. This was certainly ideal in a time when observing phenotypes was easy, and little or nothing could be done with nucleic acids. It was this kind of genetics that was performed by people like Johann Gregor Mendel in the 19th century with peas, Thomas H. Morgan in the early 20th with Drosophila, and more recently, by Barbara McClintock with maize.

Then, progress in molecular biology allowed the identification of physical genes, and to translate this “genetic logic” into a molecular picture. The first molecular step was certainly then to identify the mutated genes responsible for the phenotypes. However, the development of molecular biology also led to the identification of many genes with unknown function. And one of the best ways to definitively identify a function for a gene is to find out what happens when it is inactivated… leading to the concept of reverse genetics. But reverse genetics requires having a means to selectively mutate a chosen gene. In many cases (mostly in yeast and mammalian systems), the inactivation of the chosen genes was accomplished by homologous gene targeting, resulting in a complete (or sometimes conditional) knockout of the gene.

Thus, the need for reverse genetics has certainly driven most of the homologous gene targeting experiments in ES cells. Today, the outcome of genome sequencing has provided a new emphasis for reverse genetics: there are thousands of identified genes, without any known function. …and we need to knockout, alter, mutate, inactivate and cripple a huge number of genes, just for the purpose of looking at the resulting phenotypes.

Thus we need more than ever efficient tools for reverse genetics, or more precisely, efficient tools for gene inactivation, such as homologous gene targeting for example.