DNA Fingerprinting and Forensics

DNA Fingerprinting Principles - Identity Unlocked

• Concept: A technique to identify individuals at the molecular level using unique DNA patterns.
• Pioneer: Sir Alec Jeffreys in 1984, revolutionizing forensic science.
• Basis - DNA Polymorphism: Variations in DNA sequences among individuals.
  • These variations are primarily in non-coding regions (introns).
  • VNTRs (Variable Number of Tandem Repeats): Also known as minisatellites. Comprise repeat units of 10-100 base pairs. Highly polymorphic.
  • STRs (Short Tandem Repeats): Also known as microsatellites. Simpler, with 2-6 base pair repeat units. Currently more common in forensics.
• Inheritance: DNA patterns are inherited from biological parents (50% from each).
• Uniqueness: Each person's DNA fingerprint is unique, except for identical (monozygotic) twins.

⭐ The original DNA fingerprinting method developed by Sir Alec Jeffreys utilized multi-locus probes targeting VNTRs, creating a complex, barcode-like pattern.

Forensic DNA Techniques - Lab Detectives

• Core Principle: Analyzing unique DNA variations (polymorphisms) for individual identification.

• Key Markers: Specific DNA regions for profiling.

  • STRs (Short Tandem Repeats): Highly variable, short, repeated DNA sequences in non-coding regions. Gold standard. 📌 PCR & STRs: Partners in Crime (Solving!)
    • Typically 13-20 core STR loci (e.g., CODIS) used for high discrimination.
  • mtDNA (Mitochondrial DNA): Maternally inherited. For degraded samples (old bones, hair shafts) due to high copy number.
  • Y-STRs: Paternally inherited (Y-chromosome). For male DNA in mixed samples (sexual assaults).

• Workflow:

• Techniques:

  • PCR (Polymerase Chain Reaction): Amplifies minute DNA quantities.
  • Capillary Electrophoresis (CE): Separates amplified STR fragments by size.
  • RFLP (Restriction Fragment Length Polymorphism): Older method, requires larger DNA amounts, less common.
  • SNP (Single Nucleotide Polymorphism) Typing: Emerging for ancestry/phenotype.

⭐ STRs are the workhorse of modern DNA forensics, with 13-20 core loci typically analyzed to generate a highly discriminating DNA profile.

Applications & Legalities - Justice Jigsaw

• Core Forensic Uses:
  • Crime Scene Analysis: Matching biological evidence (blood, semen, hair, saliva, skin cells) to suspects or victims.
  • Identification:
    • Victims in mass disasters (e.g., plane crashes, natural calamities).
    • Missing persons.
  • Exonerating the innocent.
• Beyond Crime Scenes:
  • Paternity/Maternity Testing: Resolving parentage disputes.
  • Immigration: Verifying claimed biological relationships.
  • Wildlife Forensics: Identifying species, combating poaching & illegal trade.
• Legal Standing (India):
  • Admissibility: Indian Evidence Act, 1872 (Sec. 45 - expert opinion; Sec. 112 - paternity).
  • Sample Collection: CrPC Sec. 53 & 53A empower medical exam & DNA collection from accused.
  • Chain of Custody: Critical for evidence integrity; meticulous record from collection to court.
• Ethical & Database Considerations:
  • DNA Technology Bill: Proposes DNA Data Banks (e.g., NDNAD).
  • Key Concerns:
    • Privacy violations & potential data misuse.
    • Informed consent for sample collection, storage & database inclusion.
    • Familial searching ethics & risks of genetic surveillance.
    • Accuracy & challenges in interpreting complex/degraded DNA samples.

⭐ Locus of Amelogenin gene (AMELX, AMELY) is commonly used for sex determination in forensic DNA typing.

High‑Yield Points - ⚡ Biggest Takeaways

• DNA fingerprinting relies on Variable Number Tandem Repeats (VNTRs), unique to each individual.
• PCR is crucial for amplifying small DNA samples in forensic cases.
• Short Tandem Repeats (STRs) are the current standard, used in databases like CODIS.
• Key applications: paternity testing, crime scene analysis, and identification.
• RFLP is an older method; Southern blotting is a related detection technique.
• The Amelogenin gene helps in sex determination from DNA samples.