Crime science: Separating the DNA of identical twins

HUDDERSFIELD, England, April 23 (UPI) -- Professor Plum did it -- in the study with the lead pipe. Or was it Professor Plum's identical twin? Previously, it was too hard and too expensive to separate the DNA of identical twins, but new research has offered hope to forensic scientists.

Prosecution is often dropped in cases largely reliant on DNA evidence when the suspect is an identical twin. The risk of convicting the wrong twin is too great.

The chance of a DNA match between two unrelated individuals is extraordinarily small -- one in a billion. For siblings, the chance is 1 in 10,000. But identical twins have essentially the same DNA sequence, making the identification of the forensic evidence they leave behind extremely difficult.

But researchers at the University of Huddersfield recently developed a cost-effective and accurate method for differentiating between the genetic profiles of identical twins. The method looks at DNA methylation, a biochemical process that helps manage gene expression -- turning genes on and off.

As identical twins age, different environmental factors affect their genomes, or the ways in which their genetic material is expressed. These differences can be seen in their corresponding DNA methylation. Researchers developed a way to compare DNA methylation using a process called "high resolution melt curve analysis," or HRMA.

"What HRMA does is to subject the DNA to increasingly high temperatures until the hydrogen bonds break, known as the melting temperature," lead scientist Dr. Graham Williams, the head of Huddersfield's Forensic Genetics Research Group, said in a press release. "The more hydrogen bonds that are present in the DNA, the higher the temperature required to melt them."

"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," Williams added.

The process isn't perfect. Young twins with similar environments may not have developed significant differences in their DNA methylation. The technique also requires a large genetic sample, which may not be recoverable at every crime scene.

Other techniques for identical twin DNA differentiation exist. DNA sequences can be analyzed for mutations, and compared against each other. But this process is time-consuming and expensive.

The new technique is detailed in the journal Analytical Biochemistry.