DNA Testing in Sexual Offences

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DNA Testing in Sexual Offences - Code of Life Clues

  • Principle: Compares suspect's DNA with evidence from victim/scene.
  • Sources: Semen, blood, saliva, hair (with root), epithelial cells, vaginal swabs.
  • Types of DNA:
    • Nuclear DNA (nDNA): Unique to individual (except identical twins).
    • Mitochondrial DNA (mtDNA): Inherited maternally; useful for degraded samples/hair shafts.
  • Techniques: While PCR-based STR analysis remains standard, massively parallel sequencing (MPS) offers improved capabilities for analyzing low-quantity or degraded DNA samples and distinguishing between monozygotic twins.
  • Significance: Identification, exclusion, linking individuals, establishing contact under BSA provisions.

⭐ Alec Jeffreys is known as the father of DNA fingerprinting.

  • Chain of Custody: Crucial for admissibility under BSA framework. 📌 Standard procedures involve detailed documentation of collection, packaging, sealing, preservation, transportation, and analysis, ensuring unbroken accountability record per BNSS protocols.

DNA Testing in Sexual Offences - Trace, Tag, Testify

  • Trace (Sources of DNA):
    • Semen (spermatozoa, seminal fluid: Acid Phosphatase, p30/PSA)
    • Saliva (e.g., bite marks, oral swabs)
    • Blood (victim's, assailant's)
    • Hair (plucked, with root/sheath)
    • Epithelial cells (skin, vaginal/anal/oral swabs, fingernail scrapings)
  • Tag (Collection & Preservation):
    • Use sterile Sexual Assault Evidence Collection Kit (SAECK).
    • Air-dry all moist evidence (swabs, clothes) before packaging.
    • Package items separately in labelled paper bags/envelopes.
    • Rapid drying is critical to prevent degradation. While 4°C is acceptable for short-term storage, freezing at -20°C or -80°C is preferred for long-term preservation.
    • Meticulous Chain of Custody (CoC) is vital.
  • Testify (Analysis & Legal Value):
    • Standard: PCR-based STR (Short Tandem Repeats) analysis remains widely used.
    • Massively Parallel Sequencing (MPS/NGS) increasingly adopted for degraded/low-quantity samples with enhanced capabilities.
    • Yields DNA profile for comparison (suspect, victim, CODIS).
    • Strong corroborative evidence: links or excludes.

⭐ The two-finger test is banned and unscientific; DNA evidence is preferred for corroboration in sexual assault cases under BNS provisions.

DNA Testing in Sexual Offences - Unraveling the Strands

DNA fingerprinting is pivotal in sexual offence investigations, linking suspects to crime scenes or excluding innocents with high accuracy.

  • Principle: Exploits unique hypervariable regions in an individual's DNA (except identical twins).
  • Common Sources:
    • Semen (vasectomized males shed cells), blood, saliva
    • Vaginal, anal, oral swabs
    • Fingernail scrapings, plucked hair (root), skin cells
  • Core Techniques:
    • PCR (Polymerase Chain Reaction): Amplifies minute/degraded DNA.
    • STR (Short Tandem Repeat) Analysis: Gold standard; examines 20 specific, variable loci (expanded from original 13 in 2017).
    • Y-STR Analysis: For male DNA in mixed samples (e.g., azoospermic assailant).
    • mtDNA Analysis: For hair shafts, old bones; maternally inherited.

⭐ Combined DNA Index System (CODIS) uses a standard set of 20 STR loci (expanded from 13 as of January 1, 2017) for creating DNA profiles.

  • Significance: Provides powerful associative evidence; crucial for corroboration and justice. Exonerates the wrongly accused.

DNA Testing in Sexual Offences - Justice by Genes

  • Role: Links suspect to victim/scene via unique genetic profile in sexual assaults.
  • Legal Framework (India):
    • BNSS: Sec 53, 211 (DNA from rape accused), 164A (victim exam).
    • BSA: Sec 12 (identity), Sec 39 (expert opinion).
  • Admissibility: Strong corroborative evidence; expert opinion (Sec 39 BSA).
  • Samples: Blood, semen, saliva, hair, skin. Chain of custody crucial.
  • Analysis: PCR-based STR profiling standard; NGS/MPS for complex cases.
  • Challenges: Contamination, degradation, mixed DNA, interpretation.
  • Ethics: Consent (victim), privacy, data security.

⭐ Under Section 211 BNSS, an accused of rape can be examined by a medical practitioner, which includes collection of samples for DNA profiling, even without consent if necessary as per law.

High‑Yield Points - ⚡ Biggest Takeaways

  • DNA evidence is paramount in sexual offense cases, primarily from semen, saliva, or blood.
  • Short Tandem Repeats (STRs) are the most commonly analyzed DNA markers.
  • Maintaining a strict chain of custody is vital for legal validity.
  • Proper sample collection and preservation (e.g., air-drying, paper bags) prevents DNA degradation.
  • DNA can link a suspect to the crime or exonerate the innocent.
  • Consent is generally required for reference sample collection from individuals involved.
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Practice Questions: DNA Testing in Sexual Offences

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Which test is most sensitive for detecting semen in forensic investigations?

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_____ assay is used to test a glycoprotein present in semen, blood and urine of a male

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_____ assay is used to test a glycoprotein present in semen, blood and urine of a male

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DNA Testing in Sexual Offences - Free Indian Medical PG