Have we finally cracked the identical twin code? Heat treatment reveals even the most subtle differences in the DNA of siblings

  • Identical twins start life with the same genes from the same fertilised egg
  • As they age, the number of hydrogen bonds in the DNA changes due to different life experiences such as smoking and different diets
  • By heating DNA until bonds break, researchers can spot these differences
  • This will help identify which twin is guilty in a criminal case, for example   

In the era of advanced DNA profiling, police can usually identify a suspect with a single strand of hair or a tiny drop of blood.

Their techniques, however, fall apart if the suspect has a twin.

Identical twins share 100 per cent of their genes, meaning that until now it has been impossible to tell which sibling a sample comes from.

But the problem has been solved by a team of British researchers.

Identical twins share 100 per cent of their genes, meaning that until now it has been impossible to tell which sibling a sample comes from. But researchers at the University of Huddersfield have created a technique which they claim can distinguish between the genetic fingerprints of identical twins (illustrated)

Identical twins share 100 per cent of their genes, meaning that until now it has been impossible to tell which sibling a sample comes from. But researchers at the University of Huddersfield have created a technique which they claim can distinguish between the genetic fingerprints of identical twins (illustrated)

Researchers at the University of Huddersfield have created a technique which they claim can distinguish between the genetic fingerprints of identical twins.

The breakthrough may eradicate a stumbling block which has often meant it was impossible to tell which of the pair was guilty of a crime.

IDENTIFYING TWIN DNA STRAINS 

Identical twins start life with the same genes because they come from the same fertilised egg which divides into two embryos.

But as they age different life experiences, such as smoking or diet, create variations in the number of hydrogen bonds in the DNA.

Using a technique called high resolution melt curve analysis - or HRMA - researchers can now identify the difference between each twin.

HRMA subjects 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.

In 2013, 22-year-old twins Mohammed and Aftab Asghar were both charged with raping a 17-year-old woman in Berkshire because the police were unsure which one DNA evidence came from.

Mohammed was eventually convicted by a jury at Reading Crown Court, and charges dropped against Aftab because mobile phone evidence placed him elsewhere.

But in some cases it has even led to prosecutions being dropped, rather than run the risk of convicting the wrong twin.

Murder charges were dropped in 2011 against Orlando Nembhard, 19, accused of shooting a teenager outside a nightclub in Arizona, the US, because prosecutors could not prove that he was the murderer rather than his twin, Brandon.

Dr Graham Williams, who leads the Forensic Genetics Research Group at Huddersfield, alongside Leander Stewart and their colleagues have now come up with a method which could solve such riddles. 

Identical twins start life with the same genes because they come from the same fertilised egg which divides into two embryos. But as they age their different life experiences create a variation in the number of hydrogen bonds within DNA (illustrated). This could be caused by smoking, exposure to chemicals, or different diets

Identical twins start life with the same genes because they come from the same fertilised egg which divides into two embryos. But as they age their different life experiences create a variation in the number of hydrogen bonds within DNA (illustrated). This could be caused by smoking, exposure to chemicals, or different diets

Dr Graham Williams (pictured), who leads the Forensic Genetics Research Group at Huddersfield, found that by heating DNA until its bonds break these subtle differences become apparent. The technique is based on the concept of DNA methylation - the molecular mechanism that turns various genes on and off

Dr Graham Williams (pictured), who leads the Forensic Genetics Research Group at Huddersfield, found that by heating DNA until its bonds break these subtle differences become apparent. The technique is based on the concept of DNA methylation - the molecular mechanism that turns various genes on and off

Method is called high resolution melt curve analysis (HRMA). The more hydrogen bonds present in DNA, the higher the temperature needed to melt them

Method is called high resolution melt curve analysis (HRMA). The more hydrogen bonds present in DNA, the higher the temperature needed to melt them

They have created a method of heating DNA until its bonds break - identifying minute differences between two twins.

Identical twins start life with the same genes, because they come from the same fertilised egg which divides into two embryos.

But as they grow older their different life experiences create a variation in the number of hydrogen bonds within the DNA.

This could be caused by smoking, sunshine, exposure to chemicals, or different diets.

Dr Williams, whose research is published in the journal Analytical Biochemistry, says that using a technique called high resolution melt curve analysis - or HRMA - can identify the difference between each twin.

He said: ‘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.’

The technique is based on the concept of DNA methylation - the molecular mechanism that turns various genes on and off.

Environmental factors alter the methylation status of DNA, changing the way it works.

In 2013, twins Aftab and Mohammed Asghar (pictured left and right) were both charged with raping a woman in Berkshire because the police were unsure which one DNA evidence came from. Mohammed was eventually convicted and charges dropped against Aftab because mobile phone evidence placed him elsewhere

In 2013, twins Aftab and Mohammed Asghar (pictured left and right) were both charged with raping a woman in Berkshire because the police were unsure which one DNA evidence came from. Mohammed was eventually convicted and charges dropped against Aftab because mobile phone evidence placed him elsewhere

TWIN DNA IN CRIMINAL CASES  

In 2013, 22-year-old twins Mohammed and Aftab Asghar were both charged with raping a 17-year-old woman in Berkshire because the police were unsure which one DNA evidence came from.

Mohammed was eventually convicted by a jury at Reading Crown Court, and charges dropped against Aftab because mobile phone evidence placed him elsewhere.

But in some cases it has even led to prosecutions being dropped, rather than run the risk of convicting the wrong twin.

Murder charges were dropped in 2011 against Orlando Nembhard, 19, accused of shooting a teenager outside a nightclub in Arizona because prosecutors could not prove he was the murderer rather than his twin, Brandon.

For example, if one twin has a job outdoors and the other a desk job, the methylation profile will be slightly different.

Crucially, methylation changes the number of hydrogen bonds.

Dr Williams said: ‘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.’

The technique has some limitations, Dr Williams acknowledges.

For example young twins, or twins raised in highly similar environments, may not have yet developed sufficient methylation differences.

The technique also requires a relatively large sample of DNA, which might not always be present at the crime scene.

‘Nevertheless, we have demonstrated substantial progress towards a relatively cheap and quick test for differentiating between identical twins in forensic case work,” Dr Williams said.

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