Forensic scientist finds a way to separate DNA of identical twins in a crime scene

Twins share 99.99% of genetic information. Image is for representative purposes only.
Twins share 99.99% of genetic information. Image is for representative purposes only.

In January 2009, a heist was staged in Kaufhaus des Westens, a luxurious seven-story department store universally known as KaDeWe and a prominent Berlin landmark. Jewellery worth $6.8million was stolen and the surveillance camera caught three masked thieves getting away from the scene of crime, outsmarting the security systems. DNA evidence found from a drop of sweat on a latex glove discarded in the crime scene was analyzed and they got two hits. It was that of two 27-year old identical twins who were surprisingly released even before the case went to trial. Reason: From the evidence, they couldn’t clearly deduce who had committed the crime, as identical twins share 99.99% of their genetic information. Since they couldn’t be detained indefinitely without solid proof, they were released. Sounds like those perfect crimes in Hollywood movies? You bet.

However, such acquittals may become a thing of the past as Dr. Graham Williams from the Forensic Genetics Research Group at the University of Huddersfield has developed a solution to the problem and published their findings in the journal Analytical Biochemistry.

Existing methods to distinguish twins involve ‘mutation analysis’, where the genome of the twins are sequenced to identify mutations that may have occurred to one of them. “If such a mutation is identified at a particular location in the twin, then that same particular mutation can be specifically searched for in the crime scene sample.  However, this is very expensive and time-consuming and is unlikely to be paid for by cash-strapped police forces,” according to Dr Williams, who has shown that a cheaper, quicker technique is available.

It is based on the concept of DNA , which is effectively the molecular mechanism that turns various genes on and off. As the twins grow older, environmental factors which may differ between them such as smoker/non-smoker, outdoor job/desk job etc will cause changes in the methylation status of their DNA. The team proposed a technique named “high resolution melt curve analysis” (HRMA).

“What HRMA does is to subject the DNA to increasingly high temperatures until the hydrogen bonds break, known as the melting temperature.  The more hydrogen bonds that are present in the DNA, the higher the temperature required to melt them,” explains Dr Williams.

“Consequently, if one DNA sequence is more methylated than the other, then the melting temperatures of the two samples will differ – a difference that can be measured, and which will establish the difference between two identical twins.”

Limitations include insufficient methylation differences incase the twins are raised in similar environments and large sample amounts required for analysis. “Nevertheless, we have demonstrated substantial progress towards a relatively cheap and quick test for differentiating between identical twins in forensic case work,” says Dr Williams.

Read more: Press release