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The Forensic Biology Section of the Maine State Crime Laboratory analyzes biological material recovered as evidence at crime scenes. The initial examination of evidence, including the location and identification of biological material and stains, is often conducted in the Forensic Chemistry Section and then transferred to the Forensic Biology Section for DNA analysis. The DNA profiles obtained can be compared to suspects and victims in the case, and DNA profiles of supposed perpetrators can also be entered into and searched against the CODIS database, within the state of Maine and across the United States through the National DNA Index System at the FBI Laboratory. The state CODIS database also contains DNA profiles of unidentified remains, missing persons and their relatives, as a way to help locate and identify human remains that might otherwise remain unknown. The Forensic Biology Section also provides assistance to the Chief Medical Examiner’s office in identifying decedents using DNA analysis if the individuals cannot be identified using the more traditional techniques of fingerprints or dental restoration.
DNA, or deoxyribonucleic acid, has three properties that make it a powerful investigative tool. First, DNA has the ability to be replicated through an enzymatic process called polymerase chain reaction (“PCR”), enabling forensic scientists to produce many copies of DNA from only trace amounts of biological material recovered at a crime scene. This greatly increases the sensitivity of DNA analysis, enabling the crime lab to obtain DNA profiles not only from minute amounts of blood, semen, or saliva, but also from the DNA left behind by simply touching an object. Second, DNA has the ability to carry copious amounts of information, and every nucleated cell in an organism contains the complete genomic profile, regardless of what type of cell the DNA is extracted from, as long as it has a nucleus. This allows a person to be matched to their DNA profile from blood, semen, hair root, skin, saliva, etc. Third, every person’s DNA is unique, except for identical twins. This allows matches to be made to an individual to the exclusion of the vast majority of all other people, often generating random match probabilities of billions to one. These three properties enable the Maine State Police Crime Laboratory to create a unique and individual profile from the DNA left in biological material recovered from a crime scene, victim, or suspect, and compare that DNA profile to known references to either include or exclude individuals as potential contributors of the biological material. Exclusions are absolute; if the DNA profile from evidence does not match the known reference, it could not have come from that individual. If the DNA profile is the same as the known reference, it can often be said that with the exception of identical twins or close relatives, it is concluded to a reasonable degree of scientific certainty that the DNA from an item came from that person. The profile is created from 13 different locations, or loci, in the autosomal DNA that do not code for any physical characteristics. It is the natural variation within these loci, which can only be seen or detected through DNA analysis, which would eliminate someone as a potential donor if the biological material was in fact from someone else. The fact that the DNA at these 13 loci is non-coding is important because DNA analysis is not used to identify the physical appearance of a suspect, but only to include or exclude a suspect as the perpetrator of a crime based on his or her DNA profile. Additionally, it is possible to obtain male DNA profiles from the non-autosomal Y-chromosome DNA in evidence, even if it has overwhelming amounts of female DNA present, which would normally obscure any autosomal male DNA. The downside to Y-STR profiling is that all male relatives in a family will have the same Y-STR profile, so a single individual can never be identified as the sole source of a Y-STR DNA profile. Also, because of the non-recombinant nature of inheritance between paternally related males, there are no databases of Y-STR profiles, so the technique can only be utilized in cases with known suspects that can be compared directly to the evidence profile.
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