DNA Profiling

DNA Basics & Sources - Code of Life ID

• DNA (Deoxyribonucleic Acid): Unique genetic blueprint for individual ID (not identical twins).
  • Structure: Double helix. Nucleotides: phosphate, deoxyribose sugar, nitrogenous base (A,T,C,G).
  • Base Pairing: A-T, G-C.
• Polymorphisms: DNA sequence variations enabling identification.
  • Key Markers: STRs (Short Tandem Repeats) in non-coding regions - highly variable.
  • VNTRs (Variable Number Tandem Repeats) are longer, also used.
• Sources (Nucleated cells):
  • Blood (WBCs), Semen, Saliva.
  • Hair (with root), Tissues, Bones, Teeth.
  • Trace DNA: Skin cells (touch DNA), urine (epithelial cells).
  • 📌 Mnemonic (Common): Blood, Semen, Saliva, Hair root, Tissue, Teeth (BSSHTT).
• Sample Integrity: Crucial; prevent contamination & degradation.

⭐ Sir Alec Jeffreys developed DNA fingerprinting in 1984.

DNA Profiling Techniques - Gene Detective Gear

Core methods to identify individuals using their unique DNA signatures.

• PCR (Polymerase Chain Reaction): Amplifies tiny DNA amounts. Foundation for most modern techniques.
• STR (Short Tandem Repeats) Analysis: Current gold standard. PCR amplifies short, highly variable DNA regions (alleles).
  • Fragments separated by capillary electrophoresis.
  • Basis for DNA databases (e.g., CODIS).
• RFLP (Restriction Fragment Length Polymorphism): Largely obsolete method in modern forensic DNA analysis. Uses restriction enzymes to cut DNA into variable fragments. Requires larger, intact DNA samples and is time-consuming compared to current techniques.
• Mitochondrial DNA (mtDNA) Analysis: For degraded samples (e.g., old bones, hair shafts without roots). Maternally inherited; high copy number.
• Y-STR Analysis: Targets STRs on the Y-chromosome. Male-specific; traces paternal lineage. Useful in sexual assault cases with mixed samples.
• SNP (Single Nucleotide Polymorphism) Analysis: Detects variations at single base pair positions. Useful for highly degraded DNA or when STR analysis fails.

⭐ The Combined DNA Index System (CODIS) uses 20 core STR loci for human identification in the USA (increased from the previous 13 loci).

Applications & Legal Aspects - Evidence That Sticks

• Criminal Cases:
  • Link suspect to crime scene/victim (e.g., blood, semen, hair, saliva).
  • Exonerate innocent individuals.
  • Identify victims: mass disasters, missing persons.
• Civil Cases:
  • Paternity/Maternity disputes (rebuts Sec 106 BSA presumption).
  • Inheritance claims.
  • Immigration verification.
• Legal Framework (India):
  • Admissible as expert opinion (Sec 39, Bharatiya Sakshya Adhiniyam, 2023).
  • Sample collection from accused (Sec 31, 32 BNSS); consent often debated.
  • Crucial: Unbroken chain of custody, proper sample collection, preservation & storage.
  • DNA Technology Regulation Bill: Proposed framework awaiting parliamentary passage.

⭐ In India, DNA evidence is admissible under Section 39 of the Bharatiya Sakshya Adhiniyam, 2023, as expert opinion.

Ethical Considerations in DNA Profiling - Handle With Care

• Informed Consent: Essential before sample collection; individual must understand purpose, use, and implications.
• Privacy & Confidentiality: DNA data is highly sensitive; robust measures needed to prevent unauthorized access/disclosure.
• Data Security: Risk of misuse of genetic information; secure storage and access protocols are critical.
• Genetic Discrimination: Potential for discrimination in employment or insurance based on genetic predispositions.
• Familial Searching: Raises concerns about genetic surveillance and privacy of relatives.
• Sample Retention: Policies for storage duration and destruction of DNA samples/profiles must be clear.

⭐ The DNA Technology (Use and Application) Regulation Bill aims to create a national DNA database for specific categories of offenders and missing persons.

High‑Yield Points - ⚡ Biggest Takeaways

• DNA profiling (DNA fingerprinting) relies on Variable Number Tandem Repeats (VNTRs), especially Short Tandem Repeats (STRs).
• Polymerase Chain Reaction (PCR) is crucial for amplifying small or degraded DNA samples.
• Ideal sources include blood, semen, saliva, hair roots, and body tissues.
• Major applications: paternity disputes, individual identification (living/deceased), and crime scene investigation.
• Mitochondrial DNA (mtDNA) is used for highly degraded samples and tracing maternal lineage.
• Y-STR analysis is specific for male DNA and paternal lineage; CODIS is a key DNA database system.