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Genetic Variation
Genes are the functional unit of inheritance. Genes are DNA elements which reside at a specific location (locus) within the human genome and includes regions involved in regulation of expression and regions that code for a specific functional products (mostly proteins).
Nature does not tolerate a lot of variation within genes since changes, or mutations, can result in genetic disease, if the mutation disrupts the function of a protein, or cancer, if the changes are in regulatory elements. As important as genes are for medical genetics and cancer, they only account for approximately 2% of the human genome.
At least 50% of the human genome is made of simple repeat sequences. Because these regions of our DNA do not encode proteins or regulatory element, it appears that changes can accumulate with impunity.
Genetic elements that are highly variable between individuals are called polymorphic (many forms). Although they are not genes (we'll call them genetic elements), they do reside at a specific locations (loci), within the genome.
Most forensically important loci are tandem repeats. Tandem repeats are DNA elements that are repeated, head to tail, many times. The number of repeats is what varies between individuals.
Fig. 1 Tandem Repeats. The arrows represent 7 identical elements that are repeated, head to tail.
Variable Number Tandem Repeats (VNTR) have repeat units of 9 to 80 bp. Depending on the number of repeat units, the genetic elements can 1 to 20kb in size. VNTR loci are typically dectected by RFLP.
Another importent class of repeating elements are the Short Tandem Repeats (STR). STRs have repeat elements that are 2-5bp in length. Most STR alleles contain between 5 and 45 repeating elements and range in size from 20-180bp (the actual size will be larger because of the inclusion of flanking sequences). Because of their relatively small size, STR loci are well suited to PCR amplification.