Thanks, perhaps, to the amount of crime drama we now consume as entertainment, the layperson today appears to believe that DNA evidence in a case is clinching — it can make or break a case. But unclouded by what we see on our screens, what is the reality regarding the probative value of DNA in establishing guilt or innocence?
In mid-June, the Madras High Court set aside the conviction of a man who had been accused of rape in a POCSO case. The bench of Justices M. S. Ramesh and Sunder Mohan J. found merit in an appeal by the man, who argued that though it was subsequently proved that the victim falsely accused him of raping her, the DNA test establishing his paternity had dealt the blow, leading to his conviction. The judges held that the prosecution had not proved the case against the appellant beyond reasonable doubt, and therefore, set the conviction aside.
But that was not all. The judgement also examined, in detail, the wisdom of relying on DNA evidence alone to establish guilt. The facts in the case were thus: The victim had originally stated that the appellant had committed penetrative sexual assault on her, however, in her deposition, she said she had blamed the appellant, as she did not want her love affair with another man to come into the open for fear of criticism by relatives. The prosecution witness 1 too turned hostile, and in these circumstances, it would not have been possible to render a finding of guilt against the appellant, his counsel argued in court. Thus, the conviction was based on the DNA report, which compared blood samples collected from him and the victim’s child. The report stated that the cumulative probability of the paternity of the appellant being the father of the child (of the victim) was 99.999999998%.
How DNA profiling works?
DNA is deoxyribonucleic acid, and it is genetic material present in the nuclei of cells in living organisms. Justices Mohan and Ramesh relied on an article published by the Central Forensic Science Laboratory, Kolkata, explaining this in detail. An average human body is composed of about 100 trillion cells. DNA is present in the nucleus of a cell as a double helix, supercoiled to form chromosomes along with intercalated proteins. Twenty-three pairs of chromosomes are present in each nucleated cell, and an individual inherits 23 chromosomes from the mother and 23 from the father transmitted through the ova and sperm, respectively. All information about internal organisation, physical characteristics, and physiological functions of the body is encoded in DNA molecules in a language (sequence) of alphabets of four nucleotides or bases : Adenine (A), Guanine (G), Thymine (T), and Cytosine (C), along with the sugar phosphate backbone.
Since the same DNA sequence is present in every cell of the body (apart from mature red blood cells) therefore, DNA can be sourced from any biological material. This includes saliva, semen, vaginal fluids, blood, body tissues, teeth, hair, and bones. The quantity of DNA contained in biological material varies. Blood and saliva are richer sources of DNA as compared to teeth and hair roots, which are DNA deficient. DNA may be left behind on objects during physical contact, which is commonly referred to as touch DNA or trace DNA. Touch DNA contains very low amounts of DNA and is not an ideal source for DNA profiling, explains a paper in the Forensic Science India Report (2013–2017), which looks at the science underlying forensic DNA profiling and its value in criminal proceedings, notably admitting that there are inherent limitations to the process.
There is apparently much to say about the method for the collection, packaging, storage, and transportation of a DNA sample. Though it differs based on the source of the biological material and the conditions in which it is found, the idea is to ensure that the quality and quantity of DNA are preserved. There are well defined techniques for both the collection and transportation of various kinds of samples to prevent contamination and ensure the quality of the sample extracted.
How reliable is DNA?
But the moot question is: how reliable is DNA profiling in criminology? The basic fact we need to know is that the DNA in all human beings is 99.9% identical. In forensic DNA profiling, a few locations in the remaining 0.1% of DNA are chosen to create a person’s DNA profile. These specific locations at which the DNA is examined are called loci. These loci have repetitive sequences of DNA called Short Tandem Repeats, or STRs. While every individual has the same sequence, or STR, on a locus, the number of times that it repeats itself may vary across individuals. This variation in the number of repetitions is referred to as alleles. At a given locus, an individual has two alleles, one inherited from the father and the other from the mother. The current genetic markers of choice for forensic purposes are STRs. STRs present on the Y chromosome are used in sexual assault cases or to determine paternal lineage.
DNA profiling is complicated, and each sequential step involved in generating a profile can vary depending on the facilities available in the laboratory. The analysis principles, however, remain: isolation, purification and quantitation of DNA, amplification of selected genetic markers, visualising the fragments and genotyping, statistical analysis and interpretation, the Kolkata lab explainer adds.
Manikanda Raj, head of forensic medicine and toxicology at Chengalpattu Medical College and Hospital, says DNA analysis as a process is based on probability and, in that sense, cannot be considered conclusive evidence. There are definite pitfalls, he says, particularly with sample contamination, where there is a delay in lifting the sample or testing it, when the sample becomes unviable, or the samples themselves are switched. “Then the tests turn out inconclusive. One way to ensure at least the right samples are tested is to videograph the entire process – something we have been following in Tamil Nadu — thanks to a directive by Justice P.N. Prakash.” He’s very clear that convictions cannot be made only on the strength of a DNA analysis, without other corroborating evidence.
How conclusive is DNA profiling?
The Madras High Court, in its verdict in the case. also explained possible outcomes, based on scientific resources. There are three possible outcomes of a profile comparison: (1) Match : If the DNA profiles obtained from the two samples are indistinguishable, they are said to have matched. (2) Exclusion : If the comparison of profiles shows differences, it can only be explained by the two samples originating from different sources. (3) Inconclusive : The data does not support a conclusion.
It also quotes a Law Commission of India report, ‘A Review of the Indian Evidence Act, 2003’, which observed that: “If the samples match, that does not mean the identity is conclusively proved. Rather, an expert will be able to derive from a database of DNA samples an approximate number reflecting how often a similar DNA “profile” or “fingerprint” is found. It may be, for example, that the relevant profile is found in 1 person in every 1,00,000 : This is described as the “random occurrence ratio”. Thus, DNA may be more useful for purposes of investigation but not for raising any presumption of identity in a court of law.”
In Pattu Rajan v. State of T.N. 2019, judges considered the value to be attached to a DNA report: “Like all other opinion evidence, the probative value accorded to DNA evidence also varies from case to case, depending on facts and circumstances and the weight accorded to other evidence on record, whether contrary or corroborative. This is all the more important to remember, given that even though the accuracy of DNA evidence may be increasing with the advancement of science and technology with every passing day, thereby making it more and more reliable, we have not yet reached a juncture where it may be said to be infallible. Thus, it cannot be said that the absence of DNA evidence would lead to an adverse inference against a party, especially in the presence of other cogent and reliable evidence on record in favour of such party.”
