Mutation is a change in the hereditary material of an organism’s cells. By altering this material, a mutation changes certain traits. Some mutations produce obvious changes. For example, the variety of grape called Concord is the result of a mutation. This mutation caused a wild grapevine to produce grapes that were bigger and sweeter than before. Mutations may be transmitted to future generations.
Hereditary material consists of genes and chromosomes. Genes, which are composed of a substance called deoxyribonucleic acid (DNA), determine the hereditary traits of an organism. The genes are lined up along the chromosomes, which are microscopic threadlike bodies. A mutation can affect an individual gene or an entire chromosome. A gene mutation occurs if there are slight chemical changes in DNA. Sickle cell anemia is a blood disease caused by a gene mutation. In a person with this disease, a minor change occurs in the DNA of a gene that controls the production of red blood cells. A chromosome mutation occurs if the number or arrangement of chromosomes changes. Down syndrome is a mental and physical disorder caused by a chromosome mutation. The disorder occurs if a person is born with an extra copy of a certain chromosome.
Scientists do not know what causes most mutations, even though these changes occur at known rates. Some mutations are caused by such agents as ultraviolet light, X rays, and certain chemicals. Others are caused by transposable elements—certain segments of DNA that can change position within the chromosomes. Agents that cause mutations are called mutagens. The type of mutation that will be caused by a mutagen cannot be predicted.
An organism can pass a mutation on to its offspring only if the mutation affects cells that produce eggs or sperm. This type of mutation is called a germinal mutation. The other type of mutation, called a somatic mutation, occurs in other cells of the body.
A mutation may or may not have a visible effect on the organism that carries it. Most mutations that cause a visible change are harmful. However, some of these mutations enable an organism to survive and reproduce better than other members of its species. Such beneficial mutations—if they are germinal—are the basis of evolution. If the mutant organism passes a beneficial trait on to its offspring, they also will have an advantage in survival and reproduction. After many generations, most members of the species will have the trait.
Breeders use mutations to produce new or improved species of crops and livestock. They do this by breeding certain plants and animals that have one or more favorable mutations. These plants and animals are called sports.
