Before the 1980s, courts relied on testimony and eyewitness accounts as a main source of evidence. Notoriously unreliable, these techniques have since faded away to the stunning reliability of DNA forensics. In 1984, British geneticist Alec Jeffreys of the University of Leicester discovered an interesting new marker in the human genome. Most DNA information is the same in every human, but the junk code between genes is unique to every person. Junk DNA used for investigative purposes can be found in blood, saliva, perspiration, sexual fluid, skin tissue, bone marrow, dental pulp, and hair follicles (Phillips, 2008). By analyzing this junk code, Jeffreys found certain sequences of 10 to 100 base pairs repeated multiple times. These tandem repeats are also the same for all people, but the number of repetitions is highly variable. Before this discovery, a drop of blood at a crime scene could only reveal a person’s blood type, plus a few proteins unique to certain people. Now DNA forensics can expose a person’s gender, race, susceptibility to diseases, and even propensity for high aggression or drug abuse (Phillips, 2008). More importantly, the certainty of DNA evidence is extremely powerful in court. Astounded at this technology’s almost perfect accuracy, the FBI changed the name of its Serology Unit to the DNA Analysis Unit in 1988 when they began accepting requests for DNA comparisons (Lewis, 1989).
There are thirteen standard tandem repeats used in modern forensics, and together these sequences create a DNA profile. Except in the case of identical twins, the probability that two people have the same genetic code at all thirteen core loci is less than one in one trillion (Crest, 2005). Investigators compare these genetic fingerprints with profiles stored in databases of previous offenders, and if they find a match, it proves that the person was at the crime scene. DNA forensics can also narrow down suspect pools, exonerate innocent suspects, and link crimes together if the same DNA is found at both scenes. However, without existing suspects, a DNA profile cannot direct an investigation because current knowledge of genotype-phenotype relation is too vague for DNA phenotyping. For example, a profile from a first time offender that has no match in any database may give the information that the criminal is a left handed male of medium stature with red hair and freckles. It would be impossible to interview every man who fits that description. However, with available suspects, DNA forensics has many advantages over other forms of evidence. One is the longevity of DNA. Although it will deteriorate if exposed to sunlight, it can remain intact for centuries under proper conditions (Silverstein, 1996). Because DNA is so durable, investigators can reopen old cases to reexamine evidence.
DNA from animals and plants can also be utilized in criminal forensics. One of the most common applications of this is the analysis of pet hair from a crime scene, which often links its owner to the crime. DNA fingerprints have also been applied to cannabis plants, and a database is being created to trace samples to their sources. This has been extremely successful so far, as this technology can distinguish between closely related, carefully bred plants (Westphal, 2003). Heather Miller Coyle of the Connecticut State Forensic Science Laboratory says, “It links everybody together: the user, the distributor, the grower. That’s the real intent of it, to show it’s not just one guy with a little bag of marijuana, but it’s a group of people.” (Westphal, 2003).
After a sample has been collected, it’s placed in a tube with ethanol and other chemicals that break the cells apart and release their DNA. The next step is to place the tube in a microcentrifuge that uses centrifugal force to separate the solution into layers according to their weight. The tube is then incubated at 56 degrees Celsius for a few hours and spun in the centrifuge again (Butts,...
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