New Delhi: As the aviation industry searches for answers to the Air India flight 171 crash, forensic teams on the ground are busy conducting a grim task. Teams have been collecting DNA samples to identify the dead and provide closure to grieving families.
But this is a challenge. The explosion and high temperature from the fuel fire would have destroyed soft tissue, fingerprints and even dental records. What’s more the extreme heat can destroy nuclear DNA in tissues, hair, or blood, making it hard to extract.
Mitochondrial DNA (mtDNA) in burned bones and teeth survives better than nuclear DNA, even if the body parts are charred.
“As a forensic geneticist who is involved in DNA fingerprinting, we look at whether we are getting a little bit of the DNA sequence or not. When I say little bit, even the tiniest one nanogram of DNA is enough,” said Madhusudan Reddy, Scientist, Group Head, Centre for DNA Fingerprinting and Diagnostics (CDFD), to The Print. Reddy was part of the team that collected DNA samples from the 2010 Mangalore air crash.
Mitochondrial DNA (mtDNA) is valuable in forensics because it persists even when nuclear DNA is degraded, but routine sequencing isn’t common due to limited demand and the need for specialised skills, according to a study published in the Emerging Topic in Life Sciences in 2021.
“(So) only when all the other options of collecting DNA samples run out, we use mtDNA profiling to find a match,” Akhil Benny, Assistant Professor, forensic science, Kristu Jayanti College, told ThePrint.
mtDNA is passed down only through mothers, like an unbroken chain through all maternal ancestors, Benny said.
“For this mitochondrial DNA, the samples have to be maternal relatives, like you have to have siblings or mother or someone from the maternal lineage to compare,” Reddy added.
Think of mitochondrial DNA (mtDNA) as a unique barcode inside every cell. Scientists can’t tell it apart from regular DNA at first, but they use special “DNA hooks” (primers) to grab and copy just two specific parts of the barcode, called HV1 and HV2. Then the scientists use a reference sample (from relatives) to match for the mystery sample (bones or other samples). Then they line up both copies of the HV1/HV2 to spot similarities.
In this case, scientists need to create DNA profiles of samples from the crash as well as profiles of relatives’ DNA. “(For example) in this case, there are 242 bodies, so theoretically you should get 242 unique distinct profiles,” Reddy added.
DNA profiling tough after extreme heat
At crash sites, experts look for bone samples. That said, due to the impact, chances are high that the samples can get mixed up, which makes profiling tougher. There will be mixed samples if two body parts come together. Hence, the forensic expert has to be careful to separate the mixed profiles and match them, according to Reddy.
“We may get multiple (mixed) specimens, but we can identify whether it is coming from the same person or not by using DNA profiling,” Suresh, a DNA examiner at the Regional Centre for Biotechnology (RGCB), Trivandrum, told ThePrint.
“The greatest advantage of DNA profile is that the DNA profile of the individual, wherever you get it from a leg or a hand or a head, will be the same,” said Reddy.
“So, if you get five different samples, and if they give an identical profile, you can say that they are coming from one individual.”
Time is also a challenge in such crashes. “If the sample is burnt, then it may take some time to get the DNA,” Suresh added.
As far as the bone sample is concerned, it takes a while for it to decalcify before DNA can be isolated. “It takes somewhere around 48 hours (to remove the calcium from the bone), then to isolate the DNA from the osteocytes, which are cells in the bones,” Reddy explains.
In a disaster like a passenger flight crash, as the authorities have a clear list of passengers, the DNA identification is more structured than in open-ended mass disasters like earthquakes, floods or bombings, where victim lists are unknown.
(Edited by Viny Mishra)