Understanding Genetic Mutation: Causes, Effects, and Inheritance

Mutation Browse the article Mutation

Mutation

Mutation, in a general sense, any change in the genetic makeup of an organism; such a change usually causes the organism to have a trait not possessed by any of its ancestors. In many instances the new trait can be handed down to the organism's descendants.

Organisms in which mutations occur, and their descendants that possess the new trait, are sometimes also called mutations, but are properly called mutants. Breeders of organisms, often refer to the first mutant of a new variety as a “sport.” Mutations occur spontaneously and at random in nature from causes that are not completely understood by scientists. Mutations may also be produced artificially by exposing a cell or a whole organism to radiation, heat, or certain chemicals.

Effects of Mutations

Mutations are essential to the progress of biological evolution. They provide the material that is worked upon by the forces of natural selection to produce new species and new varieties within species. Although mutations are a major factor in the development of new types of organisms, there are other factors involved as well. These include sexual recombination (the combining of genes from two parents) and geographic isolation (the separation of populations by a physical barrier).

Some mutations are of economic value. Long before the laws of heredity were discovered, breeders took advantage of the appearance of new and desirable traits in plants and animals and, by selective breeding, produced improved varieties. For example, in the late 18th century, a sheep with extremely short legs was born on a New England farm. Realizing the advantage of having sheep that could not jump fences, the farmer bred the mutant animal and produced a short-legged variety called Ancon sheep.

Breeders of fur-bearing animals such as minks and foxes take advantage of natural mutations to produce a wide range of fur colors. Most economically important plants are the result of selective breeding. The seedless grape is an example.

On the whole, however, mutations are more apt to be harmful than beneficial to the organism in which they occur. Each organism has a combination of traits that helps it adapt to its environment, making survival possible. Any random change in an efficient organism is likely to decrease rather than increase its efficiency. Geneticists estimate that 99 out of 100 mutations are harmful, and about 20 out of the 99 are lethal. Less than 2 per cent of all mutations cause visible variations, such as changes in color or shape. Invisible variations may be such important ones as infertility or the inability to live to maturity.

In 1927, Hermann J. Muller, a United States geneticist, announced his discovery that radiation can cause mutations. This knowledge added to the anxiety produced during the 1940's and afterward over the harmful effects of radioactive fallout from nuclear explosions. The genetic effects of all types of radiation and chemicals form the basis for much modern scientific research.

Gene Mutations

The general definition of mutation used at the beginning of this article includes chromosomal aberrationsas well as gene mutations. Chromosomal aberrations are mutations that affect the number or arrangement of genes on chromosomes. Gene mutations are mutations that affect the structure of genes. In modern genetics, the term “mutation” usually refers only to a gene mutation. Mutations can occur in any cell, but only those occurring in the gametes (sex cells) can be inherited.

Genes are made up of a chemical compound called deoxyribonucleic acid (DNA). The structure of DNA is altered in a gene mutation; this alteration typically occurs when a gene fails to make an exact copy of itself during the division of a cell. The term “mutant” is applied to the new type of gene, as well as to the organism in which the altered gene appears.

Reverse mutation is the only exception to the general rule that a trait created by mutation has not appeared in an ancestor. This happens occasionally when a new type of gene—perhaps generations after the first change—mutates and restores the original ancestral trait.

The frequency of gene mutations under natural conditions varies according to the species, the type of gene, and other considerations. It has been estimated that any one gene has only a million-to-one chance per generation of mutating. However, mutations may be said to occur frequently because there are thousands of genes in each cell.