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Defining STR, Locus and Allele

In this video, Dr Gavin Turbett explains the terms locus, STR and allele which are central to forensic DNA profiling.
So, in regards to forensic DNA testing, the reality is that most of our DNA is the same from one person to the next. DNA can be broadly broken into two categories there is the Coding DNA and the Non-Coding DNA. The coding regions are the instructions for the proteins that make up our bodies and there are usually only very small and subtle differences between people, and the differences in DNA are much more common in the non-coding regions. In forensic DNA testing, areas of difference in non-coding regions are specifically targeted for analysis.
They are of interest in forensics because we know they can vary, and the most common type of DNA sequence used for forensic analysis is known as a Short Tandem Repeat. So here we have a very short sequence of DNA . So, I showed you before, a little picture of DNA showing the the ladder and the rungs of the ladder. So, now we’re just here reading the sequence of the rungs of the ladder. And this is a short stretch of DNA. It’s 200 bases in length. And this is a very, very tiny fraction of your total DNA. It’s probably about 1/15 millionth part of your total genome.
Now, interestingly that sequence, while it looked random, when we look more carefully at it, we can see that there is actually a section where the DNA effectively breaks into a repetitive tract. In this example, the sequence is ACTG and that ACTG sequence has repeated itself 12 times. So that is a short tandem repeat of the sequence ACTG, and in forensics, we would refer to that as an allele 12. So, everyone will have that particular DNA sequence in our bodies, in our genomes, and we will all have the sequence ACTG at that particular location. What will differ between people is how many times that ACTG sequence actually occurs. In this example, it’s 12 times.
In someone else it might be 14 and 17. Another example, it might be different again and that is what we’re measuring, it is the number of times that that short tandem repeat actually occurs. So, forensic DNA test kits are designed to test multiple regions of DNA at once, and each area that is being tested on the DNA it’s a specific location and it’s known as a Locus and plural of that is loci. And so the test kit will test a number of loci at once, and it’s the results of testing that are the individual alleles. They are the actual results at each locus. They are the variants seen for that specific person.
Any particular forensic DNA test kit will always be testing exactly the same loci. So, the forensic DNA test kit that we are currently using in Western Australia is called PowerPlex 21. There are actually a number of forensic DNA test kits in use around the world, and this happens to be the one that we’re using at the moment. If a lab is using a different forensic DNA test kit, that is perfectly fine as long as they validated it for use and have experience with using it and getting results with it. Now, irrespective of which kit is being used, they all use the same underlying principles, and many of the loci that they test will be in common.
So even two laboratories that are using different test kits will probably find there’s enough loci in common between those two test kits that they’ll be able to make meaningful comparisons and this allows forensic DNA profile data to be exchanged and shared between laboratories and even between countries.

Short tandem repeat or STR, locus, allele, allelic table – these are terms you may come across in DNA reports. An understanding of these terms and their relation to the process of forensic DNA profiling is important while examining DNA evidence in casework. Let us understand these crucial concepts with Dr Turbett in this video.

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Decoding Forensics for Legal Professionals

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