DNA Testing in Criminal Law

The Gold Standard?

How does DNA Matching Work?

On April 14, 2003, we successfully completed the mapping of the human genome, in a landmark research work called the Human Genome Project.  It took the combined efforts of thousands of scientists around the world over 13 years, and yet there are still many things we don’t understand.

Alleles are variants of genes that have more than one form.  You might have an allele for blonde hair, or black, or brown. The rarest of all affects just 2% of the population with a recessive gene that both parents must have in order to pass along red hair.

There are over 3 billion alleles in our DNA code, but only 30 million actually code for proteins that make our bodies work.  That’s just 1% of the total.  What does the other 99% do?

“Junk DNA”

It was once thought that anything that doesn’t code for a protein must be “extra” and not particularly valuable, resulting in the contentious name “Junk-DNA”.  Through subsequent research we have learned that about 80% of our DNA is biochemically active, but have not determined how it is useful yet.  We now acknowledge that about 8-15% of the elements we once thought useless are absolutely critical for other functions to take place.

How are you “you”?

Despite the vast number of alleles or “letters” in our DNA coding, the difference between any two human beings is only about 0.1%.  Still, that is enough to uniquely identify each of us.

Interestingly, we share almost exactly the same DNA as chimpanzees and bonobos, with a difference of about 1.2%.  Even more fascinating is that you share nearly 60% of your DNA with a banana, or 90% with a cat!

Why is this?  DNA has been around for a long time, and certain functions are common to all life.  It takes millions of mutations (and years) to accidently stumble across a useful one.  Instead of reinventing the wheel every time, once a successful candidate is found, it is used until something better comes along.

Knowing Isn’t Equal to Understanding

Looking at a geographical map may tell you where a statue is located but will not tell you the reason for its existence.  If you looked at a map from the Middle Ages you might see the location of a currier or bottelier (a maker of leather bottles, similar to a traveller’s canteen) but the words alone don’t tell you anything valuable, even though it is in English (albeit, ancient English).


Among all the DNA coding are sections we call polymorphic markers.  This simply means specific identifiable sequences that change from person to person.  These areas do not code for proteins; they are simply long strings of repeating pairs.  They come in pairs because we get one from our mother and one from our father.

In one segment you might have 6 repeats from “Mum” and 14 repeats from “Dad”.  In the next segment you might have 11 and 6 from Mum and Dad, respectively.

To uniquely identify an individual, we use 20 different sites, each with two pairs of numbers like 11-6, 5-13, 10-10, and so on.  Repeats can typically range between 5 and 50.  Think of it like a key for a door lock that has 20 pins in it—and each pin can be any one of 45 different sizes.

As you can see, that is a lot of possibilities.  The actual number with 45 possibilities would be 23,188,907,000,000,000,000,000,000,000,000,000 or, easier to say, 23.2 decillions (1033).  Another way to think of it is several Sextillions (1021) of unique combinations for each person on Earth.  No chance of misidentifying one person, right?

The First Case

September 19th 1987 is a date that lives in the memories of all forensic geneticists, right alongside the otherwise forgettable name of Colin Pitchfork.  That man was the first person ever convicted in court of any crime on the basis of DNA screening and DNA evidence—and it happened in jolly old England, as they say.

That particular sociopath abducted, raped, and murdered 15-year-old Dawn Ashworth in July of 1986.  Unbeknownst to anyone, this was his second kidnapping, rape, and murder of a 15-year-old.  The first had occurred in 1983 and the victim was Lynda Mann.

The police didn’t know the depths of his depravity, for he had left his baby son in the car while he raped and killed the first girl in 1983.  He had probably used the child as bait to lure the girl in the first place.  It was revealed that after the murder that he then took his son home and put him to bed.

The Ashworth rape in 1986 was so heinous and brutal that the police sought out Alec Jeffreys, a professor of genetics at Leicester University.  Jeffreys had been using his genetic skills to determine paternity, and to help with immigration casework.

At the time the police were trying to link the murder to a 17-year-old suspect who had been bullied into falsely confessing to a crime.  With the DNA evidence police hoped to firmly tie the new crime to the young man.

They wanted Professor Jeffreys to attempt to match the killer to the boy they had in custody using the DNA samples the police had obtained from the medical examiner.  Jeffreys clearly demonstrated that the 17-year-old was not the perpetrator.

Unable to connect the current suspect, the police needed a new plan to find the real killer.  Of course, there was no huge database of DNA samples as we have nowadays.  They had to begin from scratch.

How They Did It

The county of Leicestershire holds the city of Leicester and this is the location where the kidnapping, rape, and subsequent murder of the child took place.  Back in the late 80s, the population in this region was about half a million people.

The local constabulary collected 5,000 blood and saliva samples from men between the ages of 17 and 34 in the surrounding area.  These were all painstakingly matched against the samples from the victim.

Unfortunately, nothing turned up.  Had it been a transient?  Had the killer slipped away?

Not surprisingly, though criminals may think highly of themselves, they are intrinsically foolhardy.  In relatively short order, a colleague of the murderer was overheard by an astute citizen, bragging that a man had given him money in order to visit the police and provide samples in his name.  Clearly this was an attempt to avoid the collection efforts of the local police.

The whole house of cards began to collapse, and the police ended up on the doorstep of Colin Pitchfork.  He was arrested and tested in September of 1987, and by January 1988, had been tried, convicted, and sentenced to life in prison.  Fortunately, this was a fairly short criminal career for this miscreant, all thanks to the newly created art of DNA forensics.

Losing its Lustre

Unfortunately, in the early days of genetic testing there were some significant problems.  People started regarding DNA testing as indisputable, as a Silver Bullet, or the Gold Standard.  Laboratories providing the service did nothing to caution the public about the possibility of errors creeping into the equation.

Admittedly, it was hard to resist the notion of perfection—of absolute certainty—that you would always get the right person, every time.  Since there were sextillions of combinations for every person on the planet, it would be virtually impossible to ever confuse one individual for another…or so the public thought.


Josiah Sutton, 16, and his neighbour, Gregory Adams, 19, were arrested as they were walking home one day.  41-year-old Houston woman Priscilla Stewart had seen them, and then called the police to report that she had “seen” her kidnappers.  She said these boys were the ones who had grabbed her and driven around in her Ford expedition for hours, taking turns sexually assaulting her, because she recognised their hats.

The boys, being familiar with DNA testing from the popular press offered blood samples to in their minds, get this crazy woman to go away.  Instead the lab found “credible evidence” that the 16-year-old was one of the perpetrators.

Despite no fingerprints, the fact that the boys differed significantly in appearance from Stewart’s original police report, and a complete lack of other supporting evidence, and in spite of alibis that they were elsewhere, Josiah was sentenced to prison for 25 years based on the “indisputable DNA evidence”.

His mother appealed to politicians and legal authorities to no avail.  Even the Innocence Project in New York turned her down because they didn’t deal with cases where there was DNA evidence.  Such was the power of the supposed “certainty” of DNA evidence.

Where It All Went Wrong

A lab technician named Christy Kim, working at the Houston Crime Lab, had received the blood and semen samples from the Ford Expedition, as well as the vaginal swab samples from the rape kit, and used a technique to amplify the amount of DNA present.  She compared them to the blood samples from the boys and excluded 19-year-old Adams.  She did find her “credible evidence” implicating the 16-year-old Sutton, and the rest is history.

She testified in court that the sample was distinct and that such a match would only be found in 1 of 694,000 African Americans.  Kim, using an earlier technique which has now been abandoned, found some genetic markers for the 13 sites in use at the time.  We’ve only switched to using 20 sites as of January 2017, to make a more thorough match, precisely because of cases like this.

What happened, however, was that the lab technician was untrained, using outdated technology, unsupervised, and had made many blunders as a result.  In truth, the genetic markers were not a good match, and in fact 1 in 15 African Americans could have been expected to match the results she had obtained.

Since there were two men identified, the odds where just 7.5 to 1 that one of the two men would have matched, which could be said of any randomly selected pair of black men.  Eventually, it turned out that she had run three separate samples from Josiah—one saliva, and two blood samples—and they all differed significantly from each other.

If she couldn’t even get them to match when they were from the same person, how could she possibly be expected to separate out all the constituents from a vaginal swab to identify three different people (victim, and two attackers)?

Consequently she was fired, but then reinstated after a 28-day suspension because her lawyer argued that she was following protocols dictated by the managers/supervisors.  Two supervisors resigned to avoid being fired.

And What About Josiah?

After four years in prison, working on appeals and trying to get the DNA retested, it was his mother, watching a TV show who struck gold.  The programme revealed that incompetency in DNA-testing was responsible for several wrongful convictions.  She started making phone calls and became involved with reporters from KHOU TV’s news reporting team who were investigating reports about shoddy results from the Houston crime lab.

Once the TV station reported on the Josiah Sutton case, the Houston police finally had the DNA evidence retested.  He was excluded from the suspect list.  A judge ordered him released in March of 2003.

Sadly, due to systemic problems in the U.S. legal system, particularly regarding people of colour, all the legal bodies worked very hard to dismiss the charges but only if there was no admission of innocence.  It took a year almost to the day for the state Governor, Rick Perry, to grant a pardon of innocence.  It took another year and a half for them to admit he was entitled to compensation.

He received $118,000 in compensation, which seems trivial for four years in a hostile prison system.  It also included a monthly annuity of $1,700, so if he lives to age 80, that would be $1.23 million.  Other cases have resulted in multi-million dollar one-time pay-outs.

The Takeaway

These systemic failures have caused a lot of other “sure things” like fingerprinting, ballistics, and bite marks to be questioned.  There have been numerous instances where evidence went awry.

For example, a partial fingerprint recovered from a 2004 train bombing in Madrid (Spain) was tied to a U.S. ex-soldier, who had become a lawyer after he left the army.  He was arrested, held for two weeks, and then found innocent, followed by a multi-million dollar lawsuit.

Bite marks, long disputed by legal counsel, have now been declared too subjective, or “not science” and they are not admissible as evidence in most sensible courtrooms.  Bite marks are usually recorded on skin, which is a deformable material that can also swell, shrink, or heal, so it is intrinsically unreliable.

Similarly, ballistics has experienced problems with computer-matching a manufacturer’s bullet fired in their own model of gun, up to 38% of the time according to a Fox News Report from 2002.  However, when a bullet from a different manufacturer is used, the matching failed 62% of the time.

In DNA testing, we now use more sophisticated techniques to obtain much more reliable results.  Still, DNA evidence is now considered (legally) to be insufficient on its own for a conviction.  It is included as reliable collaborative evidence, along with all the other material, of a normal court case.

It’s good evidence, and fairly reliable if the testing is done properly.  And, nowadays, it is routinely automated, eliminating the human-error factor.  It’s a good thing for the public to be aware that nothing is perfect



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