DNA, short for deoxyribonucleic acid, is the biological instruction manual found in nearly every cell of the human body, and with the exception of identical twins, each person’s DNA profile is effectively unique. For investigators, that uniqueness makes DNA one of the most powerful identification tools in modern criminal justice. Biological material such as blood, saliva, semen, skin cells, hair with tissue attached and other bodily fluids can be collected from a crime scene and analyzed in a laboratory. Technicians typically examine specific regions of the DNA known as short tandem repeats, or STRs, and use those markers to develop a genetic profile. That profile can then be compared with DNA taken from a suspect, victim or known reference sample. Investigators can also enter qualifying crime-scene profiles into databases such as CODIS, where they may be compared against profiles from convicted offenders, arrestees in jurisdictions that permit it and evidence from other unsolved cases.
DNA evidence can help identify a suspect, exclude an innocent person, connect crimes committed in different places or strengthen other evidence already developed in an investigation, but it does not automatically explain how or when biological material arrived at a scene. A match can establish that DNA is consistent with coming from a particular person to an extraordinarily high statistical degree, yet investigators still have to determine what that finding means in the context of the case. DNA can be transferred directly or indirectly, mixtures may contain material from several people, samples can degrade, and contamination or improper handling can complicate interpretation. That is why forensic DNA evidence is generally considered most powerful when it is evaluated alongside witness accounts, digital evidence, surveillance footage, fingerprints, timelines and other physical evidence. In criminal investigations, DNA is not a magic answer that solves a case by itself; it is a scientific tool that can dramatically narrow the field and help investigators determine who can, and sometimes just as importantly, who cannot, be connected to the evidence.
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