Twin trouble: Why DNA evidence isn’t always reliable

Rob Kershaw
22 January 2014

The next time you raid the cookie jar, be sure not to leave behind any hair, sweat, or skin cells. Any of these could be tested and traced back to you because every part of your body contains your DNA. However, if you happen to have an identical twin, you can relax. DNA testing is powerless when it comes to identifying which of you is guilty!

Did you know? Your pet could prevent you from committing the perfect crime! Police in the UK recently traced and convicted a killer using DNA from his cat that was found at the scene.Everything in your body is made up of tiny building blocks called cells. Every one of your cells contains your DNA: a set of unique instructions for creating you. “DNA” stands for deoxyribonucleic acid. Like other nucleic acids, DNA is a long chain of nucleotides, which are three-part molecules composed of a sugar, a phosphate, and a base.

The order of the bases in the chain of nucleotides determines what instructions are contained in your DNA. In other words, the biological difference between you and your best friend comes down to the order of the bases in your DNA—or at least some of your DNA. The order of 99.9% of the approximately three billion bases in human DNA is identical from one person to the next.

However, there are tiny differences in certain sections of the sequence that make each person unique. These differences are called genetic markers. When forensic investigators find hair or blood at a crime scene, they can extract DNA and try to match its genetic markers to other samples they have on file.

Scientists are very confident in the DNA techniques used to identify individuals. However, there is a one-in-a-billion chance that a DNA test will match two different unrelated people. By pure chance, these two people would share all of the genetic markers analyzed by the test. The chance of an incorrect result also increases if samples get mixed up or damaged.

Did you know? A DNA “lab-on-a-chip” that would help avoid human error and contamination is currently being developed. It would collect samples and generate results right at the crime scene, in as little as three hours.Things get more complicated for investigators when close relatives are involved. Relatives share more genetic markers than unrelated individuals. A test that examines more markers would be more reliable, but it would also cost more and take longer to perform. Furthermore, the DNA of identical twins is identical, which makes it impossible to determine which twin was present at a crime scene using a DNA test.

For example, during a recent New Brunswick home invasion, the victim was stabbed and one of the attackers was killed. DNA evidence, found on a mask and some gloves left at the scene, was traced to Brandon and Bradley Saia, a pair of identical twins. Unable to determine which of the twins was involved using DNA testing, the police charged both of them.

In this case, it actually turned out that both twins were guilty. Witness testimony led to the jury convicting both Brandon and Bradley of forcible entry, stealing personal property while armed with a weapon, and wearing masks while committing an indictable offence.

So, as you quietly place the lid back on the cookie jar, remember that some of your DNA has probably been left behind at the scene. But fear not! DNA test results could very well result in an unfortunate family member getting the blame, especially if you have an identical twin. Just remember that fingerprint analysis is far more accurate than DNA testing. So wear some gloves!


General information on DNA

Deoxyribonucleic Acid (DNA) (US National Human Genome Research Institute) How DNA Works (Craig Freudenrich, How Stuff Works) Introduction: What is DNA? (Scitable by Nature Education) What is DNA? (Genetics Home Reference, US National Library of Medicine)

General information on testing and forensic science

Can DNA demand a verdict? (Genetic Science Learning Center, University of Utah) Crime scene DNA testing on the move (Holly Sheahan, Royal Society of Chemistry UK) How DNA evidence works (William Harris, How Stuff Works) How does DNA testing work? (BBC Science) How Fingerprinting Works (Stephanie Watson, How Stuff Works) New Brunswick twins identical DNA made for difficult conviction in deadly home invasion (Adrian Humphreys, National Post) Saia brothers found guilty in home invasion (CBC News) University of Leicester cat DNA database to be used for solving crime (BBC News)

Scholarly publications

Liu P, et al. 2011. Integrated DNA purification, PCR, sample cleanup, and capillary electrophoresis microchip for forensic human identification. Lab on a Chip. 11(6):1041-1048.

Rob Kershaw

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