DNA Fingerprinting

By Sipra Sahoo, Media and Journal Team, Budding Forensic Club

What is DNA Fingerprinting?

DNA fingerprinting, also known as DNA profiling or genetic fingerprinting, is a molecular technique used to identify individuals based on unique patterns in their DNA. These patterns arise from highly variable segments of the genome, particularly in non-coding repetitive elements such as Variable Number Tandem Repeats (VNTRs) and Short Tandem Repeats (STRs).

Historical Background

The technique was pioneered in 1984 by British researcher Sir Alec Jeffreys at the University of Leicester. While studying genetic illnesses, Jeffreys discovered that certain DNA segments have patterns unique to individuals. This breakthrough led to the first application of DNA fingerprinting in 1986, solving a criminal case by prosecuting the perpetrator and exonerating an innocent suspect.

Principle of DNA Fingerprinting

1. Genetic Variation in Repetitive DNA

While human DNA is over 99.9% identical, specific regions—Variable Number Tandem Repeats (VNTRs) or Short Tandem Repeats (STRs)—vary significantly between individuals. These areas contain repeating DNA blocks (e.g., 2–6 base pairs for STRs) that repeat in tandem, with the number of repeats differing from person to person, creating polymorphic alleles that serve as molecular barcodes.

2. Inheritance and Uniqueness

Each individual inherits half of their genes from each parent, resulting in unique STR/VNTR profiles (except in identical twins). Testing multiple independent loci increases discriminatory power, making identical profiles in unrelated individuals extremely unlikely.

3. Amplification using PCR

Modern DNA profiling uses Polymerase Chain Reaction (PCR) to amplify targeted STR regions from minute or degraded DNA samples. Primers recognize conserved DNA areas around repeat loci, generating numerous copies for analysis, enabling precise sizing of STR alleles even from tiny samples like a hair root or a single sperm cell.

4. Fragment Separation & Profile Generation

Amplified fragments are separated using capillary or gel electrophoresis, which sorts DNA by length. The number of repeats influences fragment size, producing distinct bands for each allele. Combining results from multiple loci creates a unique DNA profile—a genetic fingerprint.

Methodology

Sample Collection & DNA Extraction: Biological material (e.g., blood, saliva, hair follicles, semen, or tissue) is collected under aseptic conditions. DNA is extracted using techniques like phenol-chloroform, silica columns, or Chelex to remove proteins and contaminants.
DNA Quantification (Optional): DNA concentration and purity are measured using UV spectrophotometry or fluorescent dye/PicoGreen assays to ensure optimal input for amplification.
PCR Amplification of STR/Loci: Specific STR (or VNTR) regions are amplified using PCR with fluorescent primers, allowing detection and sizing even from small or degraded samples.
Electrophoretic Separation: Amplified fragments are resolved by capillary or gel electrophoresis, with smaller fragments traveling faster, producing distinct peaks or bands per locus.
Detection & Visualization: Fluorescent dyes (in STR methods) or radioactive tags (in older RFLP techniques) make resolved DNA fragments visible, generating electropherograms or autoradiographs.
Profile Generation & Analysis: Fragment sizes determine repeat numbers per locus, creating a DNA profile (e.g., "6,8 at D3S1358"). Combining multiple loci results in a highly characteristic profile.
Comparison & Interpretation: Profiles are compared between samples (e.g., suspect and crime scene). Statistical models calculate match probabilities, often as low as 1 in a trillion, making matches highly discriminative.

Applications of DNA Fingerprinting

Forensic Science: Identifying suspects or victims by comparing DNA from crime scenes to known profiles.
Paternity and Maternity Testing: Establishing biological relationships through DNA profile comparison.
Medical Diagnostics: Detecting genetic disorders and disease susceptibility.
Anthropology and Evolutionary Biology: Studying genetic relationships and evolutionary patterns among populations.
Wildlife Conservation: Monitoring genetic diversity and combating poaching.
Agriculture: Developing genetically modified organisms (GMOs) and conserving crop diversity.

Applications in the Criminal Justice System

1. Linking Suspects to Crime Scenes

DNA profiles from body fluids (e.g., blood, semen, hair, or skin cells) at crime scenes are compared to individuals’ profiles. Matches provide critical evidence for convictions, as seen in the UK case where Colin Pitchfork was convicted of two rapes/murders, while an innocent suspect was cleared. The FBI’s CODIS database enables crime labs to match evidence profiles against known offenders, connecting crimes across jurisdictions.

2. Breaking Cold Cases

DNA evidence from decades-old cases can be re-examined with modern techniques, leading to breakthroughs in unsolved cases.

3. Innocence Exonerated

DNA fingerprinting has exonerated wrongfully convicted individuals by showing their profiles do not match forensic samples, as in the case of Richard Buckland, who was cleared in the Pitchfork investigation.

4. Identification of Victims and Human Remains

DNA is used to identify unknown remains in homicides, mass disasters, or missing persons cases, often by matching against relatives’ DNA, even with severely decomposed samples.

5. Exceptional and Emerging Techniques

Familial DNA Searching: Partial DNA matches in databases can identify close relatives, narrowing suspect lists (e.g., solving a 1977 case).
Non-human DNA Analysis: In a Canadian case, cat hair DNA linked a culprit to a crime scene, marking a novel use of animal DNA in trials.

Significance and Ethical Issues

DNA fingerprinting has revolutionized forensic science, medicine, and biology with its highly accurate identification capabilities. However, it raises ethical concerns regarding privacy, data protection, and potential misuse. Strict regulations are needed to safeguard individuals’ rights and prevent discrimination.

“DNA fingerprinting has changed the face of forensic science, providing a powerful tool for justice, but its use must be balanced with ethical considerations to protect individual privacy.”

Advantages of DNA Fingerprinting

Highly Accurate & Reliable: The probability of two unrelated individuals sharing the same profile is extremely low (except identical twins).
Adaptable to Sample Type: Works with blood, saliva, hair, skin, semen, or tissue, including degraded samples.
Suitable for Minute or Old Samples: PCR enables analysis of tiny or degraded DNA.
Useful in Criminal Justice: Convicts criminals and exonerates the innocent.
Helps in Kinship and Identification: Establishes biological ties and identifies victims in disasters.
Permanent Stability: DNA samples can be stored and reanalyzed years later.

Disadvantages & Limitations

Privacy & Data Abuse Threats: Stored DNA profiles risk misuse or discrimination based on health, ethnicity, or ancestry.
Procedural Error Risk: Contamination or mismanagement can lead to incorrect convictions or false positives.
Not Definitive: Results are probabilistic, and issues like null alleles or close relatives can complicate analysis.
Mixed or Degraded Samples: Multiple contributors or poor preservation can hinder profiling.
High Cost and Accessibility: Requires sophisticated equipment and expertise, which may be inaccessible in resource-poor settings.
Ethical and Legal Issues: Consent, database storage, and familial searching raise significant ethical concerns.

References

DNA Fingerprinting Forensic Science Genetics STR VNTR PCR

DNA Fingerprinting

DNA Fingerprinting

What is DNA Fingerprinting?

Historical Background