How DNA Testing Works
DNA
DNA (deoxyribonucleic acid) is the substance that contains the genetic information of living organisms. Although all humans share 99.9% of the same DNA, the remaining 0.1% is unique to each individual except for identical twins, who are 100% alike. It is this unique 0.1% of the DNA that is used to create DNA profiles.
Testing
DNA testing is the process by which individuals are identified using their DNA profiles. It uses what are called variable number tandem repeats (VNTRs), which as the name implies are repetitive sequences in the DNA that known to be highly variable between individuals.
STR Analysis
Short tandem repeat (STR) analysis is the most common form of genetic testing. A STR is when two or more nucleotides form a pattern and that pattern is repeated in adjacent DNA sequences. To create an individual genetic profile, repeats of specific sequences are identified. Over 10,000 human STR sequences have been published.
Sample
A reference sample of the individual's DNA must be obtained, usually from saliva, although blood or other bodily products may also be used. The sample does not have to be taken directly from the individual, although that is preferable for evidentiary reasons in criminal cases: DNA can be extracted from used syringes, toothbrushes, razors, etc. The reference sample is used to create a DNA profile, to which other samples are compared to see whether there is a match.
Amplification
Nuclear DNA is extracted from a sample's cells, then polymerase chain reactions are used to amplify the specific polymorphic regions desired for the STR analysis. To resolve these sequences, either gel electrophoresis or capillary electrophoresis is used.
Gel Electrophoresis
In gel electrophoresis DNA fragments are separated using a polyacrylamide gel, and an electrical field applied. The gel is scanned into a computer and visualization achieved through either silver staining before scanning or the use of fluorescent dyes.
Capillary Electrophoresis
In capillary electrophoresis DNA fragments are injected into a glass capillary filled with a polymer. An electric field pulls the DNA along the capillary, with smaller fragments moving faster. Fluorescent dyes are used to detect the fragments.
Power
The FBI uses 13 specific STR loci in their Combined DNA Index System (CODIS). The odds of two individuals who are not identical twins having the same DNA profile using these loci can be a billion to one or even higher.
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