Dr. Balding's view of DNA contamination and the Leiterman case
The BBC recently had an
article on DNA contamination that quoted two academics who have weighed in on this case, Greg Hampikian and David Dr. Balding said, "'Every crime sample that was ever collected was contaminated. Even in the most pristine conditions in a laboratory, you cannot have a DNA-free environment,' he says.
"The point is you have to allow for that to do a correct evaluation of the evidence; all of that kind of contamination just isn't a problem, as it's not going to match. The only contamination that matters is something that would have got the suspect's DNA.'"
I agree that environmental DNA exists and can be a problem. Yet there are several major problems with this position. The first is that not every electropherogram has an equal number of unassigned peaks. The second is that DNA cannot be interrogated. One cannot tell just by looking at the peaks which ones do or do not come from environmental DNA. Dr. Jason Gilder wrote to me several years ago, “One of the standard axioms of DNA typing is, the presence of a DNA profile says nothing about the time frame or the circumstances under which DNA was transferred to that item. Contamination falls into is one such instance of an issue that cannot be identified by the electropherogram alone (unless you are dealing with a control sample or other known profile).”
The third problem is that his argument leads to some odd conclusions. It seems as if the suspect must be identified before the DNA is analyzed. Only that way would we know which is the suspect's DNA and which is not. But where does that leave us when we don't know who the suspects are when we do the analysis? Does that mean that all of the DNA we find must be from the environment? A second problem situation is when a suspect is incarcerated and then a DNA test is run and points to another person. Does Balding's position imply that the other person's DNA must be environmental because the other person was not a suspect at the time? That would eliminate the possibility of a DNA-based exoneration.
Let us assume that the given piece of evidence has a mixture of two people's DNA (Mr. A and Mr. B). And let us assume that the one of them (A) was already a suspect and that the other one (B) can be excluded as a suspect on some grounds. Balding's position could be taken to mean that the DNA evidence has now strengthened the case against the suspect, Mr.A. What this ignores is the fact that both individuals DNA ended up on the item. If Mr. B's DNA came there by means other than the commission of a crime, how does one know that Mr. A's did not arrive by an equivalent route? This is not a purely hypothetical question:
Let us consider the Jane Mixer murder, originally thought to be the work of a serial killer.
Gary Leiterman's DNA was found when some items of evidence were tested many years later. Let us grant that
Leiterman should now be a suspect, even though it is unclear from Balding's position that we should. John Ruelas's DNA was also found, yet he was only four years old at the time and living in a different city from Ms. Mixer; therefore, he was not a suspect. Should we ignore Ruelas's DNA and just conclude that Leiterman did it? That would be absurd, yet it would seem to follow from Dr. Balding's argument.
EDT
Another problem with Dr. Balding's argument is that it is unclear what his interpretation of finding a second person's DNA on an item, when the second person is a forensic technician (presumably not a suspect). The
Gregory Turner case is one example: "Finally, Crown are reminded that DNA testing results are valuable only when they are accurate. In 2001, Gregory Turner was acquitted in Newfoundland of the first degree murder of a 56-year-old woman. The only substantial evidence against the accused was DNA found on the accused’s wedding ring. On the ring, DNA from another contributor, believed to be an accomplice, was also found. Determination and diligence by defence counsel ultimately uncovered that the second DNA profile belonged to a lab technician who had been working on the victim’s fingernail clippings which were stored in close proximity to the wedding ring, raising a strong possibility of primary and secondary DNA transfer and contamination."