Distinguishing between the DNA of identical twins

UK researchers have successfully tested a technique for distinguishing between the DNA of identical twins - a previously difficult task that has had legal ramifications when it comes to forensic investigation.

Identical twins present exactly the same DNA profile as each other, making it impossible to use forensic evidence to tell which of the pair is guilty or innocent of a crime. In some cases, this has led to prosecutions being dropped.

A previously proposed method for DNA differentiation is mutation analysis, where the whole genome of both twins is sequenced to identify mutations that might have occurred to one of them. As explained by Dr Graham Williams, the head of the Forensic Genetics Research Group at the University of Huddersfield, “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,” Dr Williams continued, “this is very expensive and time-consuming and is unlikely to be paid for by cash-strapped police forces.” Dr Williams’ team claims to have developed a cheaper, quicker technique, which they have published in the journal Analytical Biochemistry.

The method is called high-resolution melt curve analysis (HRMA), which subjects DNA to increasingly high temperatures until the hydrogen bonds break. Dr Williams stated, “The more hydrogen bonds that are present in the DNA, the higher the temperature required to melt them.”

How the technique works is dependent on DNA methylation - a biochemical process where a methyl group is added to the cytosine or adenine DNA nucleotides. As twins get older and are subjected to increasingly different environments - for example, one might have a job outdoors and the other a desk job - this will cause changes in the methylation status of their DNA.

“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,” Dr Williams said.

Dr Williams admitted the technique is not foolproof: not only does it require a high sample quantity (around 100 ng) that might not be present at the crime scene, but the twins must be old enough and raised in different enough environments to have developed sufficient methylation differences.

“Nevertheless,” he concluded, “we have demonstrated substantial progress towards a relatively cheap and quick test for differentiating between identical twins in forensic case work.”

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