DNA Evidence in Criminal Proceedings: Scientific Foundations and Legal Framework

 Introduction

DNA evidence has revolutionized forensic science and criminal investigations, providing unprecedented accuracy in identifying biological materials and establishing connections between suspects, victims, and crime scenes. However, the admissibility and probative value of DNA evidence in criminal proceedings requires careful judicial scrutiny of both its scientific methodology and procedural compliance.

Legal Classification: DNA as Opinion Evidence

DNA evidence constitutes opinion evidence under Section 45 of the Indian Evidence Act, and like any other opinion evidence, its probative value varies from case to case. This classification requires courts to evaluate DNA evidence with the same rigorous standards applied to expert testimony, ensuring that the scientific foundation and methodology meet established legal requirements.

Scientific Foundation of DNA Analysis

Biological Basis

Deoxyribonucleic acid (DNA) serves as the genetic blueprint present in the nuclei of approximately 100 trillion cells in the human body. DNA exists as a double helix structure, supercoiled into 23 pairs of chromosomes, with individuals inheriting 23 chromosomes each from both parents through ova and sperm. The genetic information is encoded through sequences of four nucleotides: Adenine (A), Guanine (G), Thymine (T), and Cytosine (C).

Genetic Variability and Forensic Application

While only 0.1% of DNA (approximately 3 million bases) differs between individuals, forensic scientists analyze these variable regions to generate unique DNA profiles. The current genetic markers of choice for forensic purposes are Short Tandem Repeats (STRs), with analysis of 15 STRs using automated DNA sequencers producing profiles unique to individuals, except in cases of monozygotic twins.

Specialized DNA Analysis

Mitochondrial DNA (mtDNA): Circular, double-stranded DNA of 16,569 base pairs showing maternal inheritance, particularly useful for degraded samples due to its presence in large numbers of copies^[.

Y-Chromosome Analysis: Employed to trace male lineage and resolve DNA from males in sexual assault cases, especially helpful in detecting male profiles despite high levels of female DNA or in cases involving azoospermic or vasectomized males.

DNA Profiling Methodology

Four-Step Process

The DNA profiling process involves four sequential analytical steps:

1.       Isolation, purification and quantitation of DNA

2.       Amplification of selected genetic markers

3.       Visualizing fragments and genotyping

4.      Statistical analysis and interpretation

Statistical Analysis Framework

DNA profile comparison yields three possible outcomes:

Match: DNA profiles from evidence and reference samples are indistinguishable, requiring statistical support through Random Match Probability (RMP) calculations that estimate the frequency of the particular DNA profile in a population.

Exclusion: Profile differences indicate different biological sources, providing definitive exclusionary evidence.

Inconclusive: Insufficient data prevents definitive conclusions.

Evidence Collection and Preservation Standards

Legal and Scientific Requirements

DNA evidence must be properly documented, collected, packaged, and preserved to meet both legal and scientific requirements for court admissibility. The extremely small sample sizes used in DNA analysis necessitate heightened attention to contamination issues during evidence handling.

Contamination Prevention

Contamination can occur when DNA from another source mixes with case-relevant DNA, potentially happening through sneezing, coughing, or touching contaminated surfaces after contact with mouth, nose, or face. Proper identification, preservation, packaging, and transmission protocols are essential for maintaining evidence integrity.

Judicial Standards and Quality Control

International Guidelines

Following the UK Court of Appeal decision in R v. Dohoney & Adams, courts should require:

·       Expert testimony on DNA comparisons with random occurrence ratio calculations

·       Prosecution disclosure of calculation methodologies to defense

·       Defense expert access to databases underlying the calculations

Indian Judicial Approach

The Law Commission of India observed that while DNA evidence involves genetic material comparison, matching samples do not conclusively prove identity but rather provide statistical probability data. The Commission noted that DNA may be more useful for investigative purposes than for raising presumptions of identity in court.

Quality Assurance Requirements

Courts have emphasized that DNA profile validity and reliability depend on quality control and quality assurance procedures in laboratories. The Supreme Court has noted that mere exhibition of DNA reports without examining their underlying basis renders such evidence "highly vulnerable".

Judicial Observations on DNA Evidence

Reliability and Limitations

Courts have recognized that DNA testing provides "powerful new evidence unlike anything known before," enabling determination of biological tissue matches "with near certainty". However, judicial scrutiny must examine whether DNA techniques were reliably applied by experts and consider possibilities of sample tampering during storage periods.

Procedural Safeguards

The reliability of DNA evidence depends critically on:

·       Proper sample collection and sealing procedures

·       Maintenance of chain of custody

·       Laboratory quality control measures

·       Expert testimony regarding methodology and statistical calculations

Contemporary Legal Significance

DNA testing has both exonerated wrongly convicted individuals and confirmed valid convictions, demonstrating its dual capacity for establishing truth in criminal proceedings. However, courts must balance the powerful probative value of DNA evidence against the need for procedural compliance and methodological rigor.

Conclusion

While DNA evidence represents a significant advancement in forensic science, its admission in criminal proceedings requires careful judicial evaluation of both scientific methodology and procedural compliance. Courts must ensure that DNA evidence meets established quality standards for collection, analysis, and statistical interpretation before according it probative value in determining guilt or innocence. The classification of DNA evidence as opinion evidence under existing legal frameworks provides the necessary analytical structure for judicial scrutiny while recognizing its unique scientific foundations and capabilities.

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