What is DNA?
Deoxyribonucleic Acid (DNA) is a substance that contains all genetic material and is found in every living cell. Every individual, aside from identical twins, possesses DNA that is completely unique to them. Unlike blood evidence and other commonly used types of forensic evidence, DNA that is not deteriorated is useful for an endless period of time.
How is DNA used in the criminal justice system?
DNA is used in criminal investigations to identify suspects and inform criminal convictions. DNA is also used to exonerate people who were convicted prior to the invention of modern DNA testing or in cases where DNA testing was not conducted or conducted improperly. There are two types of DNA: nDNA and mtDNA.
What is nDNA?
DNA analyses traditionally utilize nuclear DNA (nDNA), which is extracted from the nucleus of a cell. Every cell in a person’s body contains a nucleus, which contains DNA that is entirely unique to the individual. Because nDNA is unique, analysts are able to conclude that two samples are consistent with each other.
nDNA can tell you more information than mtDNA; however, it is harder to extract due to its location in the cell. Each cell has a nucleus, but if the nucleus is damaged, it can be difficult to retrieve nDNA.
What is mtDNA?
Mitochondrial DNA (mtDNA) is extracted from the mitochondria that surround the nucleus in each cell. Each cell contains up to 2,000 mitochondria, making mtDNA far more abundant than nDNA.
When forensic analysts compare mtDNA samples for a criminal investigation, there are two possible conclusions: (1) the samples are different, allowing the analyst to exclude the sample, or (2) the samples are consistent with each other. If this is the case, it is possible that the samples are from the same person, but because of the limits of mtDNA, the analyst may only conclude the samples are related.
It is impossible to link mtDNA to only one individual because mtDNA is inherited from mothers. Thus, all people on the maternal side of the family will share the same mtDNA. This means that if an analyst determines mtDNA samples are consistent, the only thing they know for sure is that the samples are maternally related.
What is a DNA “match?”
The term “match” is somewhat misleading. Saying, “the DNA is likely to have come from someone” is more accurate. Forensic analysts cannot conclude with 100 percent certainty that two DNA samples are the same because DNA testing is based on statistics. When someone uses the phrase “DNA match,” they are referring to an incredibly high probability that the DNA samples are from the same person. Typically, the level of certainty is that there is a 1 in a billion chance that the two samples are not from the same person.
Forensic science is the application of sciences to criminal investigation and law. Forensic scientists are often called as expert witnesses to testify to scientific conclusions based on evidence from crime scenes.
Juries rely heavily on forensic science to make determinations of guilt; therefore, it is important that forensic science is accurate. There is no room for error. Inaccurate forensic science, or “junk science,” perpetuates wrongful convictions.
There is a history of inaccuracy in forensic science. Some forensic sciences have been deemed unreliable while others have been completely discounted; examples include fire investigation, bite mark evidence, lead bullet analysis, pattern evidence analysis, and microscopic hair comparison.
Relevant Case: Fred Lawrence and Paul Jenkins