Eight weeks ago, Home Minister Rajnath Singh told Parliament that forensic science had proved the Government’s claims that six terrorists—not four—had attacked the Pathankot Air Force base.
Now, the case is wide open again. Because Indian and international forensic experts, who appraised the Central Forensic Science Laboratory’s still-secret report for The Indian Express, say it only shows there is male DNA in the charred remains found at the airbase — but give no indication of how many individuals the genetic material came from, nor that it came from two distinct males.
A basic grasp of the science of genetic fingerprinting can explain the controversy. The science, experts say, can make dead men speak—but in this case, that doesn’t seem to be happening.
MYSTERY #1: A GENE SALAD
FOR experts, the biggest mystery in the CFSL’s findings is this: the DNA of the two terrorists the government says were in the Airmen’s Billet seems to have become mixed up, either together, or, possibly, with that of other men. There’s no explanation in the report of how this gene salad was made.
In its report, though, the CFSL records that it could arrive at no conclusion on the identity of the charred matter, other than that it belonged to a human male. This was because the samples yielded a result for the gender genetic marker amelogenin. But no results could be determined for eight other markers since what are known as “mixed alleles” were present.
How these “mixed alleles” ended up in charred bits of human tissue, each of which, obviously, should have come from just one person, is completely unexplained in the report.
An allele is one of a pair of genes. Everyone has sets of two alleles (barring some rare conditions), one inherited from their father, and the other from their mother—located at a specific region of the chromosome. Each allele has a specific function, like determining blood type, or eye colour.
Like forensic detectives everywhere, the CFSL relied on a technique called short tandem repeat analysis, or STR—a molecular biology method used to compare units of two to thirteen nucleotides repeated hundreds of times in a row on the DNA strand. Each STR makes up part of of an allele.
For example, TATATATATA or GTCGTCGTCGTCGTC are both STRs (with A being Adenine; G, Guanine; C, Cytosine; and T, Thymine).
Each individual’s STRs are unique. The Federal Bureau of Investigation estimates that odds of identical STRs on all the 13 loci, or pre-specified regions of the chromosome it looks at, are one in a billion.
Interestingly, CFSL relied, for its work, on the widely-used AmpFlSTR Identifiler Plus kit, made by multinational firm ThermoFisher Scientific. The company’s literature states that AmpFlSTR Identifiler Plus tests 16 loci. For reasons that aren’t clear, CFSL Chandigarh tested only nine—another mystery.
MYSTERY #2: WHAT DNA?
THE second important mystery is this: the CFSL report doesn’t say how they got the genetic material the tested, and from which part of the human body. For example, in a 2003 case which involved a body completely destroyed by fire by the Italian mafia, investigators at the Laboratory of Molecular Biology in Messina, Italy, extracted DNA from a lumbar vertebra that was relatively undamaged by the fire.
The issue isn’t a trivial one, because scientists have found extreme heat does strange things to DNA.
In 2008, Nicole von Wurmb-Schwark and her co-authors at the Institute for Legal Medicine in Kiel, Germany, conducted tests on 10 cremated cadavers, and their respective minced bones—conditions similar to those of the Pathankot samples—using, among others, the AmpFlSTR Identifiler Plus kit utilized by CFSL Chandigarh.
Their findings were less than heartening. “In two cases”, they went on, “male signals were detected in samples derived from a woman, and in three instances, more than two alleles were amplified per locus”. “STR typing of cremation remains has to be considered unreliable and not suitable for forensic purpose”, they concluded.
From the CFSL’s report, we know only that they extracted DNA from “highly carbonized tissue and bone”. Assuming they ground up this material, the research of the German team gives reason to believe the results may, simply, be unreliable.
If the genetic material was recovered from inside teeth of hard bones—as in the Italian case—the results might be more robust. Smita Shrishail, and other scholars in Bangalore, in 2011 reported successfully extracting DNA from the jawbone and teeth despite exposure to temperatures of up to 850°C.
Where bodies have been exposed to temperatures of 1,000°C or higher for an hour or more, even finding teeth or bones sometimes doesn’t help. In an ongoing New Zealand case, for example, investigators have been unable to match DNA of suspected homicide victim Timothy Bosma, despite finding hollow and cracked arm bones and a piece of tooth.