Variation occurs whenever the order of the bases in a DNA sequence changes. Variations can involve only one base or many bases. If the two strands of a chromosome are thought of as nucleotides threaded on a string, then, for example, a string can break and the order of the beads can vary. One or more nucleotides may be changed, added, or removed. In chromosomes, these changes are called polymorphisms, insertions, and deletions.

In addition to these changes, some DNA sequences called "repeats" like to insert extra copies of themselves several times. Chromosomes can also undergo more dramatic changes called translocations. These occur when an entire section of DNA on one chromosome switches places with a section on another.

Not all variations in the genome's DNA sequences have an effect. Among the variations that do cause effects, some are more serious than others. The outcome depends on two factors: where in the genome the change occurs, (i.e., in a noncoding region, coding, or regulatory region) and the exact nature of the change.