What Is Variation in the Genome?
Variation occurs whenever bases in a person's DNA sequences change. Variations can involve only one base, many bases, or even large segments of chromosomes. If the two strands of a chromosome are thought of as nucleotides threaded on a string, then, for example, a string can break, resulting in a re-ordering of the beads. One or more nucleotides may be changed, added, removed, or exchanged. In chromosomes, these changes are called polymorphisms, insertions, deletions, or translocations.
In addition to these changes, some persons have DNA sequences called "repeats" that 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 a person’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, coding, or regulatory region) and the exact nature of the change.