Devangshu Datta: To decode a killer

Last Updated: Thu, Jun 02, 2011 19:41 hrs

The Chinese Confucian code made a distinction between maternal and paternal relatives on the pragmatic basis that maternity was certain, whereas paternity was not. For similar reasons, Judaism calculates descent in matrilinear fashion.

Narayan Dutt Tiwari, who is facing a paternity suit from a 31-year-old man, may be wishing it had stayed that way. The same technology of DNA profiling that could embarrass Tiwari has been used to identify a multitude of criminals, often years after they thought they had got away scot-free.

DNA profiling makes it possible to identify not only the father of a child, but also criminals, who leave minute traces of their physical presence at crime scenes. As DNA testing methods have been refined, forensic detection techniques, as well as processes in divorce cases and paternity suits have been transformed.

DNA governs inherited characteristics like eyes, hair, height and so on. It consists of long strings of genetic material. Most cells contain complete DNA samples that can uniquely identify individuals. All DNA has multiple variable sequences of four “bases” or genetic building blocks, generally referred to as A,C,G,T in scientific shorthand.

In the case of humans, a DNA strand is packed into 23 paired bundles called chromosomes. Females possess two X chromosomes, while males possess one X and one Y.

A single sperm cell contains about three billion bases. Each egg also contains three billion bases. When sperm and egg fuse in the womb, a new and unique combination results. Identical twins have the same DNA sequences because they are conceived when a single zygote splits.

Other siblings have unique DNA, although they are easily identifiable as siblings. In 2008, DNA tests confirmed that the “missing” Grand Duchess Anastasia had been shot in Ekatarinburg in 1918, along with her parents and siblings.

It was in the early 1980s that Sir Alec Jeffreys at Leicester University, worked out how to use small DNA samples to identify people. Since then, technology has improved to a point where samples as ephemeral as a few skin cells can be used.

DNA profiling is orders of magnitude more reliable than fingerprinting. Since the Americans possessed DNA from a deceased sister of Osama Bin Laden, the body of the Al Qaeda leader could be identified with an error factor of roughly one in 18 billion.

Jeffrey’s original test looked for matches in “mini-satellites” — otherwise called variable number tandem repeats (VNTR). VNTRs – short sequences within DNA – vary and repeat almost uniquely for individuals. Even shorter micro-satellite sequences or short tandem repeats (STRs) are also unique.

The perfect test is a restriction fragment length polymorphism (RFLP) test. A DNA sample is fragmented, digested by enzymes, and the fragments examined, length by length. RFLP guarantees matches of single individuals. But it requires large, fresh DNA samples (“large” equates to a single hair) and it takes weeks to conduct.

A technique called PCR (polymerase chain reaction) testing was developed more recently. This was a breakthrough in police work. PCR is quick — the test takes a maximum of a week and it can be automated easily and cheaply. It works on very small, very old samples such as a sweat-stain on a decade-old shirt, or skin cells left behind when touching some surface. Forensic pathologist and best-selling author Patricia Cornwell suggests that PCR may even nail “Jack the Ripper”, who committed his serial killings (and wrote letters to Scotland Yard) in the 1880s.

But PCR is more hit-and-miss. It takes a very small sample of DNA, and analyses for one or two specific genes. Those genes may be possessed by many people. So, a non-match eliminates suspects. But a positive match may not directly prove anything.

As DNA testing developed, police forensic methods changed. Police departments everywhere started to train personnel to keep crime scenes uncontaminated, and to collect micro-evidence. They developed in-house forensics departments, specifically to analyse DNA. National DNA databases were set up in many countries.

In India, “civilian” use of DNA tests has exploded in the past few years with many new labs. Some offer full genome sequencing, while others only do paternity tests. This trend ties in with a rise in contested divorce and child maintenance cases. It’s not uncommon for the “father” to claim infidelity in such cases, to avoid paying maintenance. Here, DNA-testing is a killer app.

Indian police departments seem to lag civil society in use of DNA testing. There are few official DNA testing labs, and dedicated forensic departments. Evidence is referred to the few government labs that do DNA testing, and there is a backlog that further clogs up the criminal justice system.

Investigative personnel are also not trained to preserve evidence at crime scenes, much less to collect DNA samples. In the notorious Arushi Talwar case, the Centre for DNA Fingerprinting and Diagnostics (CDFD), Hyderabad rejected “touch DNA testing” for skin cells that could have nailed the killer because the entire scene was contaminated.

However, DNA testing is being increasingly used to match missing persons reports with unidentified bodies — this was the method used in the Nithari child killings. It has also been used effectively in accidents to identify bodies damaged beyond recognition. Given the increasing popularity of DNA testing in civil cases, it will presumably filter through to the criminal justice system sooner or later.

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