You will surely hear about DNA test In the news, in the crime TV series or in the police films. Often, when a murder case seems unresolved, proof that removes any doubt is precisely the DNA. This test is used since the mid -1980s in legal medicine and in courts as a scientific test for identify an individual with precision. The biological traces collected on the crime scene, such as blood, saliva, sperm or hair, become key elements for investigations. There forensic geneticsa discipline born in 1986 with a murder case in England, is also used to establish Kinan’s ties, identify victims of disasters or resolve cases of Disappeared people. DNA analysis is a tool that has revolutionized justice and legal medicine. But how do DNA tests work?
The preliminary procedures to take the DNA test
Before going into the complex laboratory analysis, a sort of “preliminary investigation” is carried out on the crime scene or on the find in question by the scientific police. This crucial step consists of careful observation to evaluate the type and quality of the biological material present. It is a matter of understanding if the track can actually provide a usable profile, resorting to specific tests and tools such as forensic lamps or luminol. Only after this initial evaluation, we proceed with the analysis of the DNA, which is divided into crucial phases:
- Extraction: DNA is isolated by the biological sample through techniques such as magnetic resins or silica membranes, eliminating impurities.
- Quantification: The amount of DNA extracted is measured to optimize the subsequent amplification.
- Amplification: The DNA of interest is multiplied through the PCR (Polyimerase chain reaction) To create sufficient copies for subsequent analyzes. The “photocopy” PCR in a targeted way a Specific section of DNAmultiplying it quickly to obtain billions of copies.
How a DNA test works and how it does not discover the genetic imprint of an individual
Each individual has a unique genetic profile, known as “genetic imprint“, With the exception of monozygotic twins, who share DNA itself as they derive from a single zigothe (the fertilized egg). Ours DNA it consists of sequences of nitrogen basesthe “bricks” of life, arranged in a specific order and identified by the letters TO, T, C And G. Although the 99.9% of human DNA is identical between people, the minimal variations Present in some areas of the genome, the whole of the genetic heritage are sufficient to distinguish an individual on the other, making each of us unique.
But how do we know where the 0.1% of the genome that varies from individual to individual and therefore allows identification? Here we meet the geneticist Alec John Jeffreyswhich in the mid -1980s discovered these regions, called polymorphisms. Without his work identifying a person from his DNA would be like finding a needle in a haystack. But thanks to its discovery and subsequent technological developments, scientists know perfectly where to go looking for these regions of DNA highly variable from person to person.
In particular, within polymorphisms there are very short sequences of nitrogen bases (a, t, c, g) which repeat a certain number of times and for this they are called Short Tandem Repeats (Strip) or microsatellites. For example, a Str in a certain chromosome can have the Tcat sequence repeated 15 times. Now, the sequences itself do not vary from individual to individual, but the number of repetitions yes. So a person can have the Tcat sequence repeated 15 times, but another may have repeated it 12 or 18 times. We can imagine a stri like a “pearl necklace” in which each “pearl” represents a sequence of nitrogen bases. Each person has the same “necklaces” but of different length, and the combination of lengths is unique and specific for that single person.
How to do a DNA test: the widespread electrophoresis
The most commonly used technique is thecapillary electrophoresiswhere the Amplified StrS via PCRs are separated in a capillary filled with gel, according to theirs size (length), and displayed via fluorescence. Automated sequencing systems allow you to “read” these sequences and determine the number of repetitions within each Str, generating a electropherogram which represents the Str lengths. At the same time, advanced technologies such as the Next Generation Sequencing are emerging, offering greater sensitivity and the ability to analyze a greater number of markers simultaneously, opening new frontiers for forensic investigations.
The important thing at this point is to have the “readings” of as many streaks. In fact, if a specific Str in the DNA that we are testing has the same number of repetitions that has that same Str in the biological sample with which we are comparing the DNA in question, this is not enough to affirm that the two DNA champions belong to the same person. But the more the stri who can compare the more this margin of doubt is reduced. For example, the FBI adopts a standard that analyzes 13 specific DNA regionscalls Loci Striin which the repeated sequences of letters vary from 5 to over 30 units. The analysis of these 13 markers provides an extremely low probability, between 1 out of 10 billion And 1 on a billion billionsthat two individuals have the same genetic profile. In practice we are talking about complete certainty.
Thanks to the progress in statistical analyzes and data collection, genetic profiles are now considered highly reliable evidence, while remaining the possibility of errors due to incomplete data or human errors.
The Innocence Project and the reversal of the investigations
A concrete example taken from a real investigation concerns Earl Washington an African American unjustly sentenced to death in 1984 In Virginia, USA, for rape and murder. At the time, DNA tests were not yet used as a judicial test, and Washington spent beyond 17 years in prison. Thanks to the intervention of the non-profit organization Innocence Projectfounded in 1992 that he devoted himself to exonerating people condemned unjustly, in the 90s Str analysis were conducted on the sperm champions found on the victim by comparing them with the genetic profile of Washington and with that of Kenneth Tinsleyalready sentenced for other crimes.
The comparison of Str profiles revealed that the DNA found on the victim perfectly corresponded to that of Tinsley, demonstrating his guilt and ‘innocence by Washington. Specifically, the results showed that Earl Washington had the following stron values: Str 1 (16, 18), Str 2 (14, 15), Str 3 (11, 12). Instead, the samples on the victim’s body and those of Kenneth Tinsley had the same number of repetitions and they fell to perfection with these values: Str 1 (17, 19), Str 2 (13, 16), Str 3 (12, 12). This case, and many others resolved by the Innocence project, demonstrates how the analysis can overturn incorrect sentences, restoring the truth and justice.
