UNDERSTANDING FORENSIC DNA AND STATISTICS
While the DNAlab, a division of Maxxam Analytics International Corporation, is the leading paternity/immigration DNA testing facility in Canada, it is also home to the premiere Forensic DNA testing facility in Canada. The forensic arm of our Forensic and DNA Services Division has been the sole private service provider of outsourced DNA testing to the RCMP Forensic Laboratory Services since 2001. Over this period, we have provided full-service body fluid testing and screening, along with forensic DNA analysis to over 350 different police agencies across Canada. During this testing, we have done body fluid identification and DNA testing for a variety of different types of crime, including:
- Sexual Assault
- Aggravated Assault
- Break & Enter
- Auto Theft
- Property Crimes
But the work doesn’t end once our forensic scientists have issued their findings and lab reports. Our forensic scientists are also routinely called upon to attend court in criminal matters to testify as a forensic DNA expert witness. One of our roles as a Forensic DNA expert witness is to help educate the judge and jury on what DNA is and how is it used as a forensic tool in criminal investigations.
WHAT IS DNA AND WHY IS IT SO USEFUL TO FORENSICS?
DNA stands for DeoxyriboNucleic Acid and it is best thought of as being the chemical blueprint of ALL living things including humans, animals, plants, fungi and bacteria. In humans, as the chemical blueprint, it will determine whether you are male or female, have brown hair versus blonde or red hair, hazel coloured eyes or brown eyes. And it will also decide whether or not you will have any medical predispositions for conditions such as cancer, diabetes or colour-blindness. Each of these characteristics is defined by the differences in sections of DNA from one person to the next. In fact, except for identical twins, no two individuals will share the same DNA. It is this ability to discriminate between individuals that makes DNA testing such a powerful tool.
One of the other attributes of DNA that is used in forensics is how it is inherited. Each person will inherit half of their DNA from their biological mother and half of their DNA from their biological father. This occurs when the father’s sperm cell fertilizes the mother’s egg giving rise to the embryo. This fertilized egg cell then gives rise to all of the cells in the human body. What that means is that every cell in the human body will have the same DNA. This allows for the comparison of the DNA from different body fluid sources. For example, we can compare the DNA from blood left at the scene of a crime to a buccal cheek swab of a suspect or even the saliva on a discarded cigarette butt to semen found on a female rape victim’s underwear.
As mentioned earlier, except for identical twins, no two individuals will share the same DNA. However, we don’t look at the entire length of a person’s DNA when doing forensic DNA analysis. Instead, we focus on areas of the DNA molecule that we know to be highly variable, or different, between individuals. The variation of each area that we examine comes from how many times a segment of DNA has repeated over and over again. These segments are called Short Tandem Repeats, or STRs. To help understand what STRs are, imagine in your mind being at a train station that has two sets of train tracks beside one another. The one train track represents the DNA you inherit from your mother while the other train track represents the DNA you inherit from your father. Now, on the one train track, there is a set of 10 connected box cars while on the other train track there is a set of 14 connected boxcars. The boxcars are identical to one another, but it is the number of boxcars or how many times they repeat along a train track that is variable. So at this particular site, you can represent it by naming it a 10,14 (where the numbers are how many times you see the repeated boxcars).
It is the same sort of scenario which plays out at different sites on your DNA molecule. But instead of repeating box cars, it is how many times a particular length of DNA is repeated along each of your two strands of DNA. While one person may be a 10,14 at one site, a different person could be a 12,13 at the same location on their DNA molecule. In our forensic DNA analysis, we take a look at 15 various sites (or train stations) on the DNA molecule with these STRs. We also look at a 16th site which helps determine whether you’re biologically male versus female.
FORENSIC DNA STATISTICS
One of two things can happen when we do a comparison between a DNA profile from a crime scene sample to the DNA profile from a suspect in the case. Either the suspect’s DNA profile will be the same as the DNA profile from the crime scene sample, OR they will be different. If the two DNA profiles are different, then the suspect is EXCLUDED as being the source of DNA left at the crime scene. However, if the DNA profiles are the same, again one of two things is possible.
- The suspect is the source of the DNA left at the crime scene; OR
- The suspect is NOT the source of the DNA left at the crime scene, but someone else is the actual source of the crime scene DNA and they happen to have the same DNA profile at the 15 sites tested.
It is this second scenario where forensic DNA statistics come into play. The statistic used in forensics in our lab is the calculation of a “Random Match Probability”, or RMP. It is an estimation of how common, or rare, a particular DNA profile is in a given population. Since we cannot determine the DNA profile of every single person who is alive or deceased or even yet to be born, we use databases to allow for the calculation of the Random Match Probability.
If you’ve ever watched CSI or any TV show that talks about DNA in court, you may have heard about the odds or chance of someone else sharing the same DNA to the order of 1 in 10 trillion, or even 1 in 10 quintillion (1.0 x 1019)…that is a 1 followed by 19 zeroes!!!! That’s a big number!
To get a better idea of how big that is, consider the average size of a grain of sand. If you were to count every grain of sand on earth, you would find there is roughly (and we’re speaking very roughly here) 7.5 x 1018 grains of sand, or seven quintillion five hundred quadrillion grains of sand.
So how do we get such significant numbers? This is where the use of reference population databases comes into play. To explain, please consider another analogy:
QUESTION: What is the chance that the next car to drive by my house will be a two-door red Honda with a broken right tail light.
ANSWER: Go to a parking lot with 200 cars and start counting!!
|Honda||1 in 5|
|Red||1 in 10|
|2-door||1 in 3|
|Right tail light broken||1 in 50|
|Combined Probability||1 in 7500|
Even though we use a reference database of only 200 cars, we can work out the probability of a 2 door red Honda with its right tail-light broken as being 1 in 7500 using only four characteristics. With forensic DNA analysis, we are looking at 15 sites on the DNA molecule. At each site, we have two pieces of information, the portion of DNA from your mother and the one from your father. So, we are actually looking at 30 different pieces of DNA information. As outlined in the analogy above, we can calculate how common, or rare, a DNA profile is in a given population (see table below).
|LOCUS||DNA PROFILE||FREQUENCY IN THE CANADIAN
|D8S1179||12, 15||1 in 32|
|D21S11||29, 31||1 in 25|
|D7S820||10, 11||1 in 7.5|
|CSF1PO||10, 12||1 in 6|
|D3S1358||15, 15||1 in 11|
|THO1||7, 9.3||1 in 8|
|D13S317||9, 9||1 in 9|
|D16S539||11, 13||1 in 110|
|D2S1338||24, 25||1 in 10|
|D19S433||12, 13||1 in 35|
|vWA||16, 17||1 in 28|
|TPOX||8, 8||1 in 10|
|D18S51||14, 15||1 in 3|
|D5S818||7, 10||1 in 19|
|FGA||21, 21||1 in 200|
|COMBINED FREQUENCY||1 in 3.5 quintillion|
Given the high variability at each of the DNA sites that we look at, we routinely see Random Match Probabilities of 1 in the quadrillions, the quintillions, the sextillions, or even the septillions (See the table below for the English names of the various powers of 10 that we’ve used when reporting DNA statistics).
|1 x 106||1 in a million|
|1 x 109||1 in a billion|
|1 x 1012||1 in a trillion|
|1 x 1015||1 in a quadrillion|
|1 x 1018||1 in a quintillion|
|1 x 1021||1 in a sextillion|
|1 x 1024||1 in a septillion|
|1 x 1027||1 in an octillion|
Now, you may have noticed that we use the Canadian Caucasian population database as a default. This often leads to the question “Why did you use the Caucasian database?? My client (the accused) isn’t Caucasian!” There is a very simple answer to that question and that is “It doesn’t matter!” The reason why is because when we have a matching DNA profile between a crime scene sample and that of a known individual (i.e. the accused), we already know that he/she has that DNA profile. It will not matter which population database we use, that individual will ALWAYS have that DNA profile. It isn’t a question any longer, it is a fact! The question now becomes, “What is the chance that someone OTHER than the accused would coincidentally happen to share the same DNA profile?” Without knowing who this other person is, or what their ethnic background is, we automatically default to the Caucasian population database as this is the largest ethnic background in Canada. If, however, we have a crime scene sample from a remote area or a community composed mainly of one ethnic background we may also present the statistical data using the other ethnic population database(s). The statistics will vary slightly between the different databases, but it will not change from 1 in a 1.0 quintillion (1 x 1018) to something as common as 1 in a million (1 x 106)!
The DNAlab’s Forensic Scientists have over 75 years of combined experience in both forensic and paternity/immigration DNA analysis and have testified in over 200 court cases across Canada! For more information on our services and how we can help, please don’t hesitate to contact us at 1-877-706-7676 (toll-free) or by sending us an email at firstname.lastname@example.org .
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