Judges around the world are involved in a debate over whether a technique known as low-copy number DNA analysis or high-sensitivity analysis, a type of DNA analysis involving the amplification of genetic material, is reliable enough to convict someone of a crime.
Forensic investigators originally needed large amounts of genetic material to decipher the DNA. Today much more sensitive tests are available. By means of polymerase chain reaction (PCR), a kind of photocopying machine for DNA, scientists can now work with smaller DNA samples. Experts can even reconstruct the entire genome using DNA fragments extracted from a single cell. This allows forensic investigators to gain usable genetic material from so-called „touch DNA“, which includes the few cells left behind when a person touches an object such as a gun or the handle of a knife.
The more sensitive DNA tests become, the more error-prone they are?
While many forensic experts are convinced that this is a powerful tool that can help solve cases, others argue that it is inconclusive and unreliable. Experts have long warned that investigators must take particular care in interpreting these tests since analyzing so few DNA molecules can lead to errors.
If stray bits of DNA, for example from a lab worker’s skin cell, contaminate the test equipment, they might get amplified along with the target genetic material. Thus, even a little contamination can skew results.
Mix of DNA
Another new problem that comes with low-copy number DNA analysis, is that a mix of DNA from more than one person can be detected. This can make it hard to distinguish the DNA relevant to a crime.
People can even leave traces of DNA on objects they haven’t touched. Jessica Goldthwaite, a staff attorney with the Legal Aid Society’s DNA unit said: „you shake my hand and then I tough a gun, your DNA could end up on the gun.“ German researches in the International Journal of Legal Medicine tested how far reaching DNA transfer can be. They rubbed a cloth on the necks of a first group of people. Afterwards a second group of people rubbed their hands on the cloth and then handled a plastic bad or a cotton cloth. In the end the scientists found DNA from the first group of people on about 40% of the examined plastic bags and cotton cloths.
If only very few DNA molecules float around in a test tube, it is possible that they do not come into contact with the PCR chemicals that are needed to replicate them. As a result, some of the original DNA may go unduplicated and the test may yield a genetic profile that does not match the source of the DNA.
Allelic drop-out is one example how unduplicated DNA can lead to errors. Each person has two copies of every gene – one from their mother and one from their father – at a given locus. The two alleles can either be the same (homozygous) or different (heterozygous). If a suspect has two different alleles of a gene and the PCR duplicates only one, a forensic scientist may conclude that both copies are identical.
So should this technique be banned for use in forensic science? „Removing it as a tool for police and prosecutors would lead to significant setbacks in 21st century evidence-gathering techniques,“ said Emily Tuttle, a spokeswoman for Manhattan District Attorney Cyrus Vance Jr. The FBI however is among the critics of the technique and forbid laboratories to run low-copy number profiles through the national DNA database.
The forensic toxicology lab at the medical examiner’s office in New York City is the only public lab in the U.S. that uses this technique to develop profiles for use in criminal cases. The medical examiner’s office has become a strong advocate for low-copy number DNA analysis. Julie Bolcer, a spokeswoman for the examiner’s office recently stated: „Low number DNA testing provides a tool for solving cases that is recognised as reliable and generally accepted by the scientific community“. Nevertheless, the technique has led to some controversies within the New York City’s examiner’s office: Marina Stajic, who worked for nearly three decades as director of the forensic toxicology lab, claims she had been forced into retirement partly because of a disagreement about the accuracy of these DNA tests.
Probably the most high-profile example of the technique and its controversy is the case of Amanda Knox, who was charged with murdering her roommate in Italy in 2007. Amanda Knox was found guilty after a low-copy number of her DNA was found on the handle of a kitchen knife that showed the victims DNA on the blade. Amanda’s boyfriend was also convicted since traces of his DNA were found on the victims bra clasp. After spending four years in prison, they were exonerated after a forensic report in 2011 called the evidence unreliable and possibly contaminated. But the controversy didn’t stop there. They were both convicted again and definitely exonerated in 2015.
At the moment there is no clear case law on the merits of the science and judges have to decide on a case-by-case basis whether or not to use DNA evidence from low-copy number DNA analysis.