Mutations are good, bad or neutral depending upon where they occur and what DNA they alter. Some mutations are both good and bad, depending on the degree to which an individual has the mutation.
A mutation is a change in a DNA sequence that alters the protein for which the DNA codes. Sometimes mutations occur due to damage from an outside source, such as ultraviolet radiation. Excess ultraviolet radiation from the sun sometimes causes mutations that lead a cell to become cancerous. Most mutations that damage a DNA strand are harmful.
Other mutations occur due to deletion or insertion of DNA base pairs. One such deletion in humans confers HIV resistance or a delayed AIDS onset. Humans who are heterozygous for the mutation, meaning they possess one mutant gene and one normal gene, experience a slower progression of AIDS. Individuals who possess two copies of the gene, or are homozygous, are resistant to HIV. Research suggests that this same mutation confers resistance to either bubonic plague or smallpox. Mutations in non-coding, or junk, DNA are neutral and have no effect on the organism.
Point mutations are mutations of a single base pair. One particular point mutation causes sickle-cell anemia. For sufferers of sickle-cell, this a harmful mutation that prevents red blood cells from transporting oxygen correctly. However, sickle-cell trait is beneficial for carriers, to whom it confers immunity to malaria.