A mutation in a gene is a structural change in the sequence of nucleotide subunits in the chains that make up DNA. Changing the structure of a gene alters the design information contained in its nucleotide sequence, and generally affects the function of that gene's product. The design instructions for that gene product are spelled out in DNA as a particular sequence of the chemical subunits called nucleotides, each of which contains a nitrogenous base: adenine (A), thymine (T), cytosine (C), or guanine (G). Hundreds of nucleotides are linked in a DNA chain in the sequence that spells out instructions for a single gene. This is analogous to conveying instructions in printed books by particular arrangements of twenty-six kinds of alphabetical letters. In the case of genes, however, there are only four letters in the alphabet.

Gene products are usually proteins, and altering the design information in a particular gene will alter the structure and function of its corresponding protein. Since proteins do all the body's work, they account for all the biological characteristics (phenotypes) of any organism. Usually, a mutation in a gene produces a harmful effect, hindering the function of the protein designed by that gene, and sometimes the hindrance is lethal to the organism. Many cancers and inherited diseases are believed to be associated with mutations. In contrast, very occasionally a mutation may be beneficial. If a beneficial mutation is inherited it could cause progeny to adapt better to their environment than their parents could. Such mutations provide a substrate for natural selection in evolving new or better biological functions.

Mutations are produced from errors when cells copy DNA, or from damage caused by radiation or chemicals. Cells contain mechanisms for repairing DNA, but they are not perfect. Changes in the nucleotide sequence can include substitution of one nucleotide for another, insertion or deletion of one or more bases, or transposition of segments of the nucleotide chain.

Although some nucleotide sequences seem more prone to mutation than others, rules governing the specific location of mutations are not evident. The view that genetic variants are produced by chance, and that natural selection favors variants that best meet the necessities of survival, led to the claim that evolution is the product of mere chance and necessity. This claim was extended theologically to assert that there is no purpose in the universe, and therefore no designer, divine or otherwise. Some challenges to this claim are based on different concepts of chance.

There are reports of mutant genes that predispose their bearers to abnormal behaviors, such as violence or addiction. A complication in the interpretation of such reports is that a behavioral gene, like most genes, would be just one of many factors determining the behavior under consideration. In addition to environment, biological history, and cultural influences, those factors would include other genes having functions coordinated with those of the behavioral gene. Nevertheless claims for the existence of such mutant genes as the socalled violence gene have provoked theological discussion about personal culpability on sin.

See also BEHAVIORAL GENETICS; DESIGN; DNA; EVOLUTION; GENETIC DEFECT; GENETIC TESTING; GENETICS

R. DAVID COLE