Things can go wrong with DNA. Individual bases or groups of bases can be changed, lost, duplicated, or rearranged. Such events are called mutations, and the resulting individuals are called mutants.

The fruit fly, Drosophila melanogaster, has been used extensively in genetics research, and different mutations have been identified for a great many of its traits. A representative pair of fruit flies is shown here. Both flies are of the wild type for all traits. Note the color of the eyes and body, and the shape of the wings. A few examples of mutations for these traits include: white eyes, black body, and vestigial wings.

Of course, mutations occur in other organisms as well, including human beings.