Fallible DNA evidence can mean prison or freedom, would you trust DNA evidence?

The introduction of DNA evidence to the courtroom in the mid 1980s revolutionised forensic science, resulting in thousands of convictions and exonerating 255 wrongly convicted people so far in the US alone. The reason for more than 50 per cent of these wrongful convictions was unvalidated or improper forensic testing, including incorrect hair, blood or fingerprint analysis.

"It's not unreasonable to hold up DNA as a way that the rest of forensic science should be done," says William Thompson of the University of California, Irvine, and an occasional expert witness on DNA. "It is better validated, and often more carefully done and more rigorously interpreted than many areas of forensic science."

That's not the same as saying DNA is perfect, however. In a growing number of cases, DNA samples taken from crime scenes produce partial profiles, partly because smaller samples are collected. "Labs are trying to get more samples and they're trying to [get results from] lower and lower amounts of DNA," says John Butler, head of the US National Institute of Standards and Technology's genetics group, which aims to improve standards in DNA testing.

A standard DNA profile consists of a series of peaks relating to the number of repeating stretches of DNA found in certain genetic sequences, or alleles (see diagram). The repeats occur at specific locations on the chromosomes, called loci, and there are two alleles at each locus- one inherited from each parent. The number of repeats in each allele varies widely between individuals, allowing a person to be identified this way. Labs in the US typically look at 13 loci, while UK labs tend to look at 10.

Yet in partial profiles, alleles may fail to show up, a phenomenon called "drop-out". False peaks in the profile created by imperfections in the analysis machine may also be mistaken for alleles. This is called "drop-in".

This year, the Scientific Working Group on DNA Analysis Methods (SWGDAM), which issues guidance to US labs performing forensic DNA analysis, published new recommendations regarding the interpretation of forensic DNA. These include a suggestion that labs develop strict criteria for deciding what denotes the presence of an allele, and what amount of DNA constitutes the minimum for a profile to be constructed. Labs should also document and define any assumptions used in the analysis of a mixture. "The bottom line is that you want to be as consistent and accurate as possible," says Butler, who chaired the SWGDAM committee.

It seems lab managers would welcome consistent rules. Forensic lab directors at the 19 labs we surveyed also provided their views about how their analysis is currently done: 15 either agreed or strongly agreed that interpretation procedures should be based on national standards, and 11 agreed or strongly agreed that decisions over alleles should not be based on analyst opinion.

Labs must also take steps to avoid bias. Butler says that some labs continue to insist upon seeing suspect profiles before analysing evidence from the crime scene, which could lead to biased decision-making (see "Crime Scene Investigation: Impartiality"). Analysts also often know too much about a suspect and other evidence to be impartial, and public labs often have close ties to police. "Crime labs, including DNA labs, should not be under the control of a law enforcement agency," says one US analyst, who wished to remain anonymous. "We are scientists, not cops or prosecutors."


Link:
http://www.newscientist.com/article/mg20727733.500-fallible-dna-evidence-can-mean-prison-or-freedom.html?page=1