99.9% of our DNA is similar to the DNA of the rest of the population. How, under these conditions, to establish the owner of the genetic material found at the crime scene? Forensic medical examination has given the answer to this question, how to distinguish two DNA.
INSTRUCTIONS. DNA provides instructions for assembling the amino acids in our proteins according to a precise sequence. For proteins to be functional, they need a certain combination of amino acids. A protein that doesn’t work or is missing can lead to a genetic disease.
Therefore, in order to distinguish between two genetic fingerprints, one must look at the remaining 0.1%, where the DNA is not known to code for anything specific, which is referred to as “non-coding DNA”. “Among the approximately 3 million DNA bases (bases form amino acids, ed.) that do not code for proteins, there are regions with multiple copies of short repeating sequences of these bases. These sequences are repeated a different number of times in different people. Such regions are called “short tandem repeats with a variable number,” and they form the basis of STR (short tandem repeat) analysis. The number of repeated sequences in these regions becomes less and less as more regions are analyzed,” explains the US National Institute of Justice.
One chance in a billion
In the early 1990s, the United States created CODIS (from the English “Combined DNA Index System”) is a database that lists DNA profiles. Forensic laboratories contribute to these databases and provide profiles of specimens collected from crime scenes, individuals convicted or arrested for a crime, and missing persons. Justice uses this database to try and match samples found during the investigation with profiles already stored in the database.
“The 13 original STR CODIS were chosen to be highly polymorphic (of which there are many variations, editor’s note) with minimal differences between populations (to be usable with any individual, editor’s note); they can be tested by PCR. and found in non-coding domains of DNA,” explains Science et Avenir Rory Rholfs, professor of molecular biology at San Francisco State University and author of a recent study published in PNAS.
With 13 STRs, the chance that someone unrelated to the actual perpetrator of the crime would fit all 13 perfectly was about one in a billion. “But they were chosen before the whole genome was sequenced. We now find that some of these STRs are not in non-coding DNA at all and point to other information about people. In 2017, 13 DNA regions were expanded to 20.
Psychiatric, neurological or dermatological diseases
Thanks to the advances made in science over the past thirty years, the genome is now known and understood much better than it was then. Several studies have begun to establish links between other non-CODIS STRs and health status. The San Francisco team wanted to study the relationship between CODIS STR and gene expression. Of the 20 CODIS markers, they found six associations between CODIS markers and neighboring gene expression in leukocyte lines of over 400 individuals that were not linked to each other in the database.