DNA

DNA profiling is universally recognized as the standard against which many other forensic individualization techniques are judged because the probabilities of a false positive are quantifiable and often miniscule. DNA is in the nucleus of virtually every cell in the human body. The history of the discovery of deoxyribonucleic acid (DNA) was not related to the criminal justice system but grew out of pure biological research. James Watson and Francis Crick discovered that the structure of the DNA was a double helix, connected by four chemical bases called adenine (A), guanine (G), cytosine (C), and thymine (T). In 1985, geneticist Alec Jeffreys discovered a way to develop a DNA profile that would lead to its use in criminal justice. Various techniques have evolved for analyzing DNA, with the most common being short tandem repeat (STR) technology that intensifies the differentiation between one profile and another. Next-generation sequencing (NGS) is the latest advancement in forensic DNA profiling and allows analysis of multiple genetic markers, resulting in a more comprehensive profile. The FBI’s Combined DNA Index System (“CODIS”) is a computer software database of DNA profiles from offenders, unsolved crime scene evidence, and missing persons. CODIS uses STR analysis at 13 different locations on the human chromosome. If the crime scene sample and the CODIS profile have matching alleles at all 13 STRs, the DNA expert makes a statistical calculation of the frequency at which that occurs in the same ethnic group in the population. That frequency provides the basis for probabilistic testimony about the likelihood that another person with the same characteristics exists in the relevant population. DNA evidence is now universally admitted by courts in the United States; however, the possibility of human error in the collection or analysis of the sample can affect its admissibility.