A tandem repeat is a sequence of two or more DNA base pairs that is repeated in such a way that the repeats lie adjacent to each other on the chromosome. Tandem repeats are generally associated with non-coding DNA. In some instances, the number of times the DNA sequence is repeated is variable. Such variable tandem repeats are used in DNA fingerprinting procedures.
Tandem repeat refers to a sequence phenomenon. What is a sequence? Really, you know the way the letters of our genome are arranged across on different chromosome. So for tandem repeats to occur, we have at least two or more of base pairs, and these base pairs are repeated in a way that they can, indeed, be unique to individuals or be more common in set populations than others. And because of this uniqueness, tandem repeats can indeed be used for fingerprinting... to remember that tandem repeats tend to occur in a part of the genome that doesn't code for protein, what we will refer to as non-coding regions.- Charles N. Rotimi, Ph.D.