DNA Fingerprinting (DNA Profiling) in Forensic Science

“DNA neither forgets nor forgives; it records every biological truth.”

1. Introduction and Historical Development

DNA fingerprinting is the most authoritative individualization technique in forensic science. It provides biological individuality with extremely high statistical certainty.

Discovery

Discovered in 1984 - By Sir Alec Jeffreys in University of Leicester, UK
First used in immigration and paternity cases

First Criminal Case

Colin Pitchfork Case (1986)
First criminal conviction using DNA evidence.

📌 UGC-NET FACT: This case also proved DNA can exonerate innocent suspects.

2. Molecular Basis of DNA Fingerprinting

2.1 Structure of DNA

Double-stranded helix
Sugar-phosphate backbone
Nitrogenous bases: A, T, G, C
Complementary pairing: A–T, G–C

2.2 Coding vs Non-Coding DNA

Type	Percentage	Forensic Value
Coding DNA	~1%	Not used
Non-coding DNA	~99%	Used for profiling

👉 Why non-coding DNA? Because it shows high polymorphism without affecting survival.

3. Genetic Polymorphism (CORE NET CONCEPT)

Definition - Occurrence of two or more alleles at a locus with frequency >1%.

Types of Polymorphism

Single Nucleotide Polymorphism (SNP)
Length polymorphism (VNTR, STR)

📌 UGC-NET FOCUS: Length polymorphism > SNP for individualization.

4. Tandem Repeats Used in DNA Profiling

4.1 VNTR (Variable Number Tandem Repeats)

Repeat size: 10–100 bp
Located in minisatellites
Used in RFLP technique

❌ Requires:

Large DNA quantity
High molecular weight DNA

4.2 STR (Short Tandem Repeats) – GOLD STANDARD

Repeat size: 2–6 bp
Located in microsatellites
Highly polymorphic
Ideal for PCR

📌 UGC-NET TRAP: STR loci are inherited in Mendelian fashion.

5. Types of DNA Used in Forensic Analysis

5.1 Autosomal DNA

Biparental inheritance
High discrimination power
Used in CODIS

5.2 Y-Chromosomal DNA

Paternal inheritance
No recombination
Useful in male–female mixed samples

❌ Cannot distinguish male relatives.

5.3 Mitochondrial DNA (mtDNA)

Circular DNA
Maternal inheritance
1000–10,000 copies per cell
Used in:

Hair shafts
Bones
Teeth

📌 NET FACT: mtDNA analysis uses sequencing, not STR typing.

6. Biological Evidence and Sample Handling

Sources

Blood
Semen
Saliva
Vaginal secretions
Hair (with root)
Bone
Teeth
Touch DNA

Collection Guidelines

Gloves mandatory
Avoid moisture
Paper packaging preferred
Maintain chain of custody

7. DNA Extraction Techniques (VERY IMPORTANT)

7.1 Organic Extraction

Phenol–chloroform–isoamyl alcohol
Produces pure DNA
Time-consuming

7.2 Chelex Extraction

Resin binds metal ions
Fast and simple
Used for PCR

7.3 Differential Extraction (HIGHLY EXAMINED)

Used in sexual assault cases to separate:

Sperm cells (male DNA)
Epithelial cells (female DNA)

📌 UGC-NET FAVORITE QUESTION: Why sperm resist lysis? → Disulfide bonds in protamines

8. DNA Quantification

Why Quantify DNA?

Prevent PCR inhibition
Avoid allele dropout
Assess degradation

Method

Real-time PCR (qPCR)

Measures:
✔ Total human DNA
✔ Male DNA
✔ Inhibition

9. PCR (Polymerase Chain Reaction)

Principle : Exponential amplification of target DNA sequence.

Components

Template DNA
Primers
dNTPs
Taq polymerase
MgCl₂

Steps

Denaturation (94–95°C)
Annealing (50–65°C)
Extension (72°C)

📌 UGC-NET FACT: PCR makes forensic DNA typing possible from trace evidence.

10. DNA Typing Techniques

10.1 RFLP Analysis

Uses restriction enzymes
Southern blotting
Autoradiography

10.2 STR Typing (MOST IMPORTANT)

Multiplex PCR
13–20 loci analyzed
Core loci:

TH01
vWA
FGA
D3S1358

📌 NET TRAP:
Heterozygous locus → two peaks
Homozygous locus → one peak

11. Electrophoresis and Detection

Capillary Electrophoresis

Separates fragments by size
Fluorescent dyes used
Output → Electropherogram

Electropherogram Interpretation

X-axis: fragment size
Y-axis: fluorescence intensity

12. Interpretation of DNA Profiles

Single Source Profile

Clear interpretation.

Mixed DNA Profile

Multiple contributors
Peak overlap
Requires probabilistic analysis

📌 UGC-NET TREND: Mixtures are the most litigated issue in DNA evidence.

13. DNA Statistics (VERY IMPORTANT)

13.1 Random Match Probability (RMP) : Probability that a random person has same profile.

13.2 Likelihood Ratio (LR)

Compares:

Prosecution hypothesis
Defense hypothesis

📌 NET FACT : Courts prefer LR over RMP in complex cases.

14. DNA Databases

CODIS

FBI-managed
STR-based
Uses autosomal loci

India

DNA Technology (Use & Application) Act
National & regional DNA databanks

15. Advanced & Emerging Techniques

15.1 Low Copy Number DNA

<100 pg DNA
Risk of stochastic effects

15.2 Rapid DNA

Automated profiling
Police-station level

15.3 Next-Generation Sequencing (NGS)

Massively parallel sequencing
STR + SNP + mtDNA together

📌 UGC-NET FUTURE TREND: NGS enables forensic phenotyping.

16. Applications of DNA Fingerprinting

✔ Criminal identification
✔ Sexual assault cases
✔ Paternity/maternity disputes
✔ Disaster victim identification
✔ Missing persons
✔ Wildlife forensics

17. Limitations and Sources of Error

Contamination
Degradation
Allele dropout
Stutter peaks
Secondary transfer

18. Ethical and Legal Issues

Genetic privacy
Consent
Database misuse
Surveillance concerns

📌 UGC-NET INTERDISCIPLINARY LINK:
Forensic science + Human rights law

19. Landmark Indian Cases

Nirbhaya Case
Rajiv Gandhi Assassination
Aarushi Talwar Case

DNA evidence played decisive roles.

20. MOST IMPORTANT UGC-NET STATEMENTS TO REMEMBER

✔ STR is the current gold standard
✔ PCR revolutionized forensic genetics
✔ mtDNA is maternally inherited
✔ Differential extraction separates sperm
✔ DNA profiling gives statistical, not absolute, certainty
✔ Identical twins cannot be differentiated by nuclear DNA

21. Conclusion

DNA fingerprinting is the most powerful, objective, and statistically validated tool in forensic science. Its ability to individualize biological material has transformed criminal justice systems worldwide.