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DNA testing: All you need to know about the science, methodology

Forensic labs test samples such as blood and fingerprints to identify people who are no longer recognisable by their physical remains.

science Updated: Oct 27, 2017 12:34 IST
Malavika Vyawahare
Malavika Vyawahare
Hindustan Times, New Delhi
Representative photo of human DNA. Blood is a dependable source to test DNA.
Representative photo of human DNA. Blood is a dependable source to test DNA.(Shutterstock)

India collecting DNA from the families of 39 Indians who were kidnapped by Islamic State in Mosul in June 2014 is a grim application of DNA science – to identify people who cannot be identified based on physical remains.

Of these samples, blood is a dependable source of DNA (Deoxyribonucleic acid). After the DNA molecule is isolated from the blood cells, millions of copies are made using a technique called polymerase chain reaction (PCR) to make it easier to analyse the genetic code contained in the DNA.

Next, stretches of the DNA are split to generate a DNA fingerprint, which is unique to each individual. These are then matched with available samples.

Head of the Laboratory of Neurospora Genetics, Centre for DNA Fingerprinting and Diagnostics in Hyderabad, Dr Durgadas P Kasbekar explains the how and why of DNA testing.

What is DNA testing used for in India?

DNA testing is finding increasing use in India, though it’s largely for checking parent/child relationships. It’s also been used to analyse DNA in semen stains and vaginal swabs in sexual assault cases, for analysing DNA in bones or teeth recovered from human remains at disaster sites to aid identification, and in sweat or spit from cloth or soil retrieved from crime scenes.

Courts, however, only admit the results of DNA tests done in government labs, though private labs also offer testing.

How reliable is the method?

DNA extracted from a non-degraded and plentiful source, such as a drop of blood taken on a filter paper in a clinical setting, is very reliable. The same sources of blood give the same results every time, so the results are very reproducible. Tooth and bone samples also give DNA. But unlike blood drops, one cannot repeatedly take teeth and bones (even from post-mortem remains) to demonstrate reproducibility.

What are the main challenges to this method of identification?

Maintaining a reliable “chain of custody” for the source of DNA is very important.

The source of DNA being minuscule and possibly contaminated with cells of other people who might have handled the source pose big challenges. For instance, an abortus might be put into formalin, presumably for tissue preservation, but the formalin makes the DNA unextractable; or if the DNA is from soft tissues that suffer rapid decomposition by bacteria and other microbes.