Last week, I looked at DNA evidence. I explained what it is and how it can be used in your story, as well as the ways in which DNA fingerprinting is not necessarily a Gold Standard with no wriggle-room for writers to inject doubt into their story.
This week, I'm going to look at some other issues that you could perhaps use in your story.

Historic and Familial DNA matches
The UK has one of the largest DNA databases in the world. Those arrested for a crime routinely have their DNA profile taken and added to the database, as do members of law enforcement. (Note: The rules over the retention of samples has changed over the years, and differ between the home nations. You may need to consider reading up on the rules to ensure accuracy.)

When a DNA fingerprint is taken, it will be compared across the database, and matches to unsolved cases can, and do, pop up. In recent years, some cold cases that pre-date DNA fingerprinting have been reopened and DNA samples taken from evidence kept in storage (Note: For murder, cases are never actually closed, even when 'solved'. The evidence gathered should not be destroyed, so you can revisit a supposedly solved case). A satisfying number of offenders who thought they got away a murder or a rape 30 or 40 years ago, have been convicted when they commit an unrelated offence (eg drink driving) and have their DNA taken.

Recently, familial DNA matches have proven successful in identifying new suspects.
As explained last week, the closer two individuals are related, the more similar their DNA will be. Identical twins share 100% of their DNA (but not their fingerprints) and it is almost impossible using current technology to distinguish which twin a DNA sample has come from. Full siblings (including non-identical twins) share 50% similarity. An individual also shares 50% similarity with each of their parents.
As the distance between two individuals on a family tree increases, the similarity between them decreases. It goes without saying that this only applies to biological relations.

In this scenario, DNA taken from a crime scene doesn't match an individual on the database, but instead partially matches a person already on the database for another reason. The police can determine that their unknown suspect was a full-sibling, or other close relation to the person already on the database, and start looking at those people as potential suspects.

How long does it take to return a DNA match?
The flippant answer is "as long as your story needs it to take".
However, you need to have some realism, so consider these following points.
Technical limits
The actual process of matching DNA fingerprinting has become faster and faster. In principal, DNA matches can be done in a few hours these days. But when is your story set? The time taken 5, 10 or 20 years ago was much longer (side note: A workmate stopped watching the X-Files in protest, when they did a DNA match overnight. This was back in 1998. The lab we worked in had a DNA sequencer capable of doing DNA matching with the correct settings and reagents - it took us days to do something similar).

At the time of writing, DNA is sent to a laboratory to be processed, it can't be done at the scene,

If you are writing a techno-thriller, you can play a bit more fast and loose with this and pretend that there are machines that can test a sample on site and give a quick answer. When Osama Bin Laden was killed, it was claimed that his identity was verified by comparing his DNA to his family members, before his burial at sea within 24 hours of his death. Given the speed with which this was done, one can probably assume that they had access to some pretty sophisticated technology.

Is there a queue?
Laboratories are typically overwhelmed with DNA samples, and so your sample may languish in a queue for weeks or months before it can be processed - the length of the backlog varies enormously, depending on jurisdiction, workload etc. Would your sample be seen as a priority, or is it just another serious crime - take a ticket and join the queue?
This is an invaluable tool for story-tellers. My DNA results come back when the story needs them to - I simply justify it in the narration. If they return very quickly, it's because they authorised the cost of a fast-track service (since the UK government abolished the world-class Forensic Science Service in 2012, most DNA testing is done by commercial firms now). If it takes a bit longer, it's because it wasn't seen as a priority or the laboratory is over-worked. Just make it sound plausible and nobody will care :-)

​How long can DNA last?
This is a question without an easy answer. In ideal laboratory conditions (stored in a freezer, dissolved in a buffer etc) you can assume that previously extracted DNA will last forever. Samples on a properly stored dress, for example semen stains from a historic rape, can also last for decades.
Improperly stored samples are more problematic (or potentially more useful for your story!). Blood-soaked clothes etc are now kept in paper evidence bags. Microbial contamination of clothing can lead to condensation being produced; in a plastic bag this potentially leads to droplets of moisture interfering with microscopic spots of blood etc. Imagine a scenario where the exact position of blood spatter on a suspect's clothing supports a sequence of events; then imagine what would happen to your case if droplets of moisture dampened and smeared those stains...

Outdoors, the elements can play a significant part. Contact DNA left behind by a person may be washed away, and strong sunlight may physically destroy the DNA. Dead bodies can remain useful for the purposes of identification for a long period of time, but if exposed to the elements, again decomposition can eventually destroy the evidence, or degrade it so that it is less easy to use. If a body is buried, then depending on how it was buried, and the soil it was buried in, DNA can be extracted for a very long-time indeed. Even if the body has been reduced to a skeleton, some genetic material may remain in the bone marrow etc. That's typically where the DNA is extracted from when scientists examine Neanderthals or other, ancient human remains. Otzi the iceman was preserved in ice for over 5,000 years, making extraction and analysis of his DNA relatively easy.

Mitochondrial DNA
If a body is very badly degraded, and a full extraction of cellular DNA impossible, then scientists can sometimes extract something called mitochondrial DNA. Mitochondria are tiny structures inside our cells that help the cells release the energy needed to carry out their functions. They carry their own, small quantities of DNA. Just like the normal DNA within cells, this will mutate over time, so that people who are more distantly related will have slightly different mitochodrial DNA to those who are more closely related. This type of DNA can also be extracted from the shaft of a hair, when the follicle is no longer attached.

The problem is that mitochodrial DNA is only passed through the female line (it is carried in the egg, not the sperm). In other words, from mother to child. This means that siblings who share the same mother will have the same mitochondrial DNA as each other and their mother, and their grandmother etc. But they won't have the same mitochondrial DNA as their father or grandfather etc.
Does this create new possibilities for your story?

I hope you have found this interesting and useful.
Please feel free to share and comment, either here or on social media.
Until next time,


Paul

Those of you of a certain age will remember the 1994 OJ Simpson trial. Mr Simpson was accused of killing his estranged wife Nicole Brown, and her friend Ron Goldman, at her home in Los Angeles. After a bizarre, slow-motion car chase, he was apprehended and tried for their murders.

The court case, filmed and shown on prime-time TV, had many memorable moments including Simpson struggling to put on a leather glove found at the crime scene.

But most notable for many was the way in which this case was one of the earliest, high-profile, uses of DNA fingerprinting. In fact much of the prosecution case rested on this evidence, and so was arguably fatally damaged when the defence placed enough reasonable doubt in jurors' minds about the reliability of the DNA evidence for them to acquit.

Much has been written about the rights and wrongs of this case, and I've no intention of rehashing it here. The aim instead is to help writers consider the importance of DNA evidence in their stories, the narrative possibilities it opens, and to debunk a few myths.
Full disclosure - I am not a forensic scientist. However I am a former molecular biologist, who worked with DNA for many years in a research setting (non-crime related). In recent years, I have taught biology to secondary school pupils.

Quick introduction to DNA (feel free to skip if you are already confident)
As tempting as it is for me to waffle on for pages about my favourite molecule, DNA, as a crime writer all you need to know are the following basic facts:
All living organisms have a unique set of instructions that tell the cells making up their body how to build the proteins they require to build themselves and carry out the chemical reactions necessary to sustain life. These instructions are written as code using a chemical called DNA. It is often referred to as genetic material.
In humans, these instructions are carried in almost all of the cells that make up our body.
We inherit half of our instructions from each of our biological parents and will pass on half of our instructions to our own kids (our partner will contribute the missing half).
Because the half of our instructions that we pass on is random each time, the precise combination of instructions that an individual receives is a unique mixture of both parents DNA, so siblings with the same parents are still genetically unique (the only exception to this rule is identical twins - non-identical twins are no more similar than regular brothers and sisters.)
Individuals that are more closely related will have more similar DNA: full-siblings share more DNA than cousins or half-siblings etc (more about this next week).

You are unlikely to need to know more details than this for your crime novel, but if you want to learn more, BBC Learning have a terrific introduction https://www.bbc.co.uk/bitesize/guides/z3mbqhv/revision/3 .

What do we mean by DNA evidence?
Back in 1984, Professor Sir Alec Jeffries, working at the University of Leicester, discovered a method of comparing DNA samples to see if they came from the same or different individuals. He called this technique DNA fingerprinting. Over the following decades, the technique has been improved so that it is quicker, more accurate and requires smaller samples of material.

Humans leave their DNA wherever they go. At a crime scene they may leave obvious samples behind - eg blood or semen stains. However as the sensitivity of the technique has increased, the amount of biological material required has decreased. Those samples can now be too small to see with the naked eye.

But a CSI doesn't need a person to leave something as obvious as body fluids behind. Here are some of the samples that they can extract DNA from.

​Epithelial cells (skin cells). We all shed skin cells to a greater or lesser degree, wherever we go. There is sufficient DNA within those cells for analysis. If you touch a surface, you may not leave a clear enough fingerprint for matching - but you can still leave traces of your DNA behind.
Hair. Head and body hairs do not have any genetic material, so cannot be tested - but sometimes the follicle, the tiny skin structure that the hair grows out of, remains attached to the hair and these cells can be tested.
Saliva. The skin cells on the inside of your mouth are constantly being replaced. Some of these cells can be found in your saliva. If a suspect bites, kisses or licks a victim, they will leave some of these cells behind.  If they bite into an apple, or smoke a cigarette, again they will leave something behind.

Former forensic technician Melissa Kreikemeier has written an excellent blog post about how good different body fluids are for extracting DNA from. Her blog is well worth a visit.

How trustworthy is a DNA match?
To put it bluntly, two good quality DNA samples can be matched with a probability measured in millions or billions to one. What this means is that the likelihood that two samples (perhaps one from a crime scene and one from a suspect) came from two different people, and just appear to have both come from the same person, is a billion-to-one chance.
Rarely these days would a barrister try to convince a jury otherwise.
So, slam-dunk right? No scope for a crime writer to credibly twist and turn the story and introduce any doubt? DNA evidence has killed the crime novel!

Not at all!

1. It needs to be a good sample: If exposed to the elements or incorrectly stored by the police, DNA will degrade over time. As the quality of the sample declines, so does the probability that the match is real.
2. It needs to be a pure sample from one individual: Until recently, if two people's DNA was mixed together - eg a victim and a suspect both bled at the scene  - then this was referred to as a mixed-profile. It was impossible to isolate one person's sample, so it couldn't be proven that the suspect was present. In recent years, advances have meant that this is no longer insurmountable, but it's still early days.
3. Contamination: As the volume of sample required for a DNA-profiling has decreased, so has the possibility of contamination. The white suits that CSIs wear, along with face masks, gloves, booties and hairnets don't just protect the technician from icky stuff, they also protect the crime scene from them. CSIs shed DNA like anyone else - as does your detective, so think about whether they would be bumbling about a scene with no protection. Perhaps more importantly, this means suspect and victim DNA can also be transferred from one scene to another. CSIs/Detectives shouldn't move between crime scenes or between the scene and interviewing a suspect without disposing of their protective equipment first. Without proper care, a victims DNA could easily end up on an innocent suspect or vice versa, establishing a forensic link where none actually exists. Cases have been thrown out when both the victim and the suspect were transported (separately) in the same vehicle. Could this be a plot point?
4. Chain of evidence: All evidence from a crime scene must be logged. Sample bags must be sealed and dated. If the bag is opened, it needs to be resealed, initialled and dated again after use. Failure to follow this procedure can and does result in evidence being deemed inadmissible, because it can't be trusted by the court. How could you use this? A way to get you out of a bind when a case is 'too easy' to solve to make a good story? Perhaps corrupt officers deliberately plant evidence at a crime scene?

The possibilities as a writer are endless, and if you think they seem implausible, OJ Simpson's defence team used three of these scenarios to cast doubt on the DNA evidence used at his trial.


Next week, I will look into some more ways that DNA evidence can be used in an investigation and ways in which you can incorporate it into your story.

As always, please feel free to chip in your own thoughts either here or on social media.


Take care,
Paul