DNA evidence constitutes opinion evidence as contemplated under Section 45 of the Indian Evidence Act, and like any other opinion evidence, its probative value varies from case to case. The courts must evaluate DNA evidence with appropriate scrutiny while considering its scientific foundation and procedural compliance.

DNA Profiling: Scientific Methodology and Process

Nature of DNA Evidence

·       DNA (Deoxyribonucleic acid) is genetic material present in cell nuclei, composed of four nucleotides: Adenine (A), Guanine (G), Thymine (T), and Cytosine (C).

·       Only 0.1% of DNA differs between individuals, with forensic scientists analyzing variable regions to generate unique DNA profiles

·       Current genetic markers of choice for forensic purposes are Short Tandem Repeats (STRs)

DNA Profiling Methodology
The DNA profiling process involves four sequential steps:

·       Isolation, purification and quantitation of DNA

·       Amplification of selected genetic markers

·       Visualizing fragments and genotyping

·       Statistical analysis and interpretation

Statistical Analysis and Interpretation Standards

Three Possible Outcomes of Profile Comparison:

·       Match: DNA profiles from two samples are indistinguishable

·       Exclusion: Profile differences indicate different biological sources

·       Inconclusive: Data insufficient for definitive conclusion

Random Match Probability (RMP): Statistical calculations must support any declared "match" by estimating the frequency of the particular DNA profile in a population.

Collection and Preservation Requirements

Legal and Scientific Prerequisites
DNA evidence must be properly documented, collected, packaged, and preserved to meet legal and scientific requirements for court admissibility. Contamination risks require heightened attention during:

·       Location and identification of biological specimens

·       Collection and packaging procedures

·       Chain of custody maintenance

·       Laboratory transmission protocols

Evidentiary Standards and Judicial Scrutiny

Quality Control Requirements
Courts have emphasized that DNA profile validity and reliability depend on quality control and quality assurance procedures in the laboratory. The mere exhibition of DNA reports without examining their underlying basis renders such evidence "highly vulnerable".

Burden of Proof Standards

·       DNA evidence may be more useful for investigation purposes than for raising presumptions of identity in court

·       Courts must examine whether DNA techniques were reliably applied by experts

·       Possibility of sample tampering during storage periods affects evidential weight

Precedential Guidelines

International Standards: Following R v. Dohoney & Adams, courts should require:

·       Scientist testimony on DNA comparisons with random occurrence ratio calculations

·       Prosecution disclosure of calculation methodologies to defense

·       Defense expert access to databases underlying the calculations

Judicial Observations: DNA testing provides "powerful new evidence unlike anything known before" and can determine biological tissue matches "with near certainty," while simultaneously serving to exonerate wrongly convicted individuals and confirm valid convictions.

Legal Principle Established

DNA evidence, while scientifically advanced and potentially conclusive, requires strict adherence to collection protocols, chain of custody procedures, and quality control standards. Courts must scrutinize both the scientific methodology employed and the procedural compliance achieved before according probative value to such evidence in criminal proceedings.
IN THE SUPREME COURT OF INDIA
Criminal Appeal Nos. 611, 612-613 and 614-615 of 2022

Decided On: 07.11.2022

Rahul Vs. State of Delhi, Ministry of Home Affairs and Ors.

Hon'ble Judges/Coram:

U.U. Lalit, C.J.I., S. Ravindra Bhat and Bela M. Trivedi, JJ.

Author: Bela M. Trivedi, J.

Equivalent Citations

2022 SCC ONLINE SC 1532,2023 SCC 1 83,2022 AIR SC 5661,2022 INSC 1176, MANU/SC/1455/2022

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