Mutation

Early geneticists treasured mutations in the organisms they studied, since no characteristic can be studied genetically unless heritable variants exist. If, for example, everyone had brown eyes, nothing could be learned about the inheritance of eye color, as all generations would have the same color of eyes. For this reason, geneticists collected and propagated all the mutants they could find, and methods were developed to deliberately induce mutations, a process called mutagenesis. Such techniques include exposing their experimental organisms to X rays and chemicals.

Transposons can also be deliberately used to introduce mutations in model organisms. In the plant Arabidopsis thaliana and in the fruit fly Drosophila melanogaster, transposon mutagenesis is often used to induce mutations, as the mutation can be very rapidly cloned and mapped with the transposon's DNA sequence as starting point.

Comparing existing mutations can help determine the evolutionary relatedness of two organisms. During evolution, there has been a relatively constant rate of accumulation of mutations in genes for a number of proteins, so the number of changes can be used to estimate the time since two species had a common ancestor. This is called the molecular clock and is illustrated in Figure 1. Each gene has evolved at a characteristic rate—the result of mutation rates, selection, and chance changes in the gene pool.

Advances in genetics have only intensified the search for mutations, especially in complex traits such as behavior and cancer, as the key to finding the genes involved and then unraveling the underlying mechanisms. This involves mapping the mutations, cloning the genes, and studying the mutants to discover what biochemical processes are changed in the mutants.

Mutations are believed to underlie most, if not all cancers. Cancer-causing mutations found so far include genes involved in communication between cells (signal transduction) and in the control of cell division. Many of these genes have been categorized into two broad classes: oncogenes and tumor suppressor genes. The mutation that has been found most often, in a tumor suppressor gene called p53, usually arises as a somatic mutation but can also be inherited as Li Fraumeni syndrome.

Xeroderma pigmentosum is an autosomal recessive condition in which the ability to repair DNA damage induced by UV light is defective. Many mutations are produced, and the affected people have large numbers of skin cancers.