A type of DNA analysis that could vastly increase the power of genetic fingerprinting has been developed by US scientists.

They have found a way of picking an individual’s DNA out of a mixed sample – even when that sample is contaminated by the DNA of up to 200 others. The method works even when the DNA of interest is only 0.1 per cent of the sample. At present, it is hard for forensic investigators to detect an individual’s DNA if it constitutes less than 10 per cent of a mixture, or if many other people’s DNA is present.

This means that it is almost impossible to identify a suspect’s DNA out of, for example, a collection of skin cells from the handrail of a public staircase. The new method could resolve this problem.

“This is a potentially revolutionary advance in the field of forensics,” said the paper’s senior author, David W. Craig, the head of the research group.

The team, from the Translational Genomics Research Institute in Phoenix, Arizona, developed a mathematical technique to determine whether the DNA of an individual was present in a mixture. They tested their method successfully on a variety of samples they had mixed.

The main interest of the researchers is the use of genetics to diagnose and potentially treat neurological conditions such as Alzheimer’s disease. As part of their work, they analyse single nucleotide polymorphisms (SNPs, pronounced “snips”) – points at which one genetic code varies from another by a single DNA letter.

The new research, published in the journal Public Library of Science Genetics, is an offshoot of this work.

SNP analysis characterises an individual’s DNA sequence, or genome, by focusing on half a million points at which one person’s DNA is most likely to differ from another.

The revolutionary element of potential interest to forensic scientists is the combination of the DNA of a group of people into a collective genome for the purpose of analysis. It is possible to calculate a mathematical measure of similarity to determine whether a single individual’s DNA is part of that group. This method of cataloguing a DNA sample is new to forensics.

Peter Gill, of the University of Strathclyde, one of the fathers of genetic fingerprinting, sounded a note of caution. “It’s clear that the test results are from pristine DNA and not from casework samples, which are usually compromised in some way,” he said.

In addition, Dr Gill explained that problems with DNA analysis in criminal cases often lie not with identifying DNA but with the issue of how that DNA came to be in a particular place.

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