A mutation is a change that occurs in the DNA sequence of an organism. Mutations can result from errors in DNA replication during cell division, exposure to mutagens or a viral infection. Mutations can occur during DNA replication if errors are made and not corrected in time. Mutations can also occur as the result of exposure to environmental factors such as smoking, sunlight, and radiation. Mutations can be inherited, particularly if they have a positive effect. Germline mutations (that occur in eggs and sperm) can be passed on to offspring, while somatic mutations (that occur in body cells) are not passed on. Mutations contribute to genetic variation within species. Mutations can also be neutral, meaning they do not influence the fitness of an individual, or they can be harmful or beneficial. Mutations in genes can have no effect, alter the product of a gene, or prevent the gene from functioning properly or completely. Mutations can also occur in non-genic regions.

Mutations can lead to genetic disorders or illnesses. For example, the disorder sickle cell anemia is caused by a mutation in the gene that instructs the building of a protein called hemoglobin. This causes the red blood cells to become an abnormal, rigid, sickle shape. However, in African populations, having this mutation also protects against malaria. Mutations can also disrupt normal gene activity and cause diseases, like cancer.

If a mutation occurs in a germ-line cell (one that will give rise to gametes, i.e., egg or sperm cells), then this mutation can be passed to an organisms offspring. These hereditary (or inherited) mutations are in almost every cell of the persons body throughout their life. Other mutations can happen on their own during a persons life. These are called sporadic, spontaneous, or new mutations. They affect only some cells. Damage from the suns ultraviolet radiation or exposure to some types of chemicals can lead to new mutations.