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: