Forensic Genetics — Pearson Alternative Academic Qualification Applied Science Revision
Forensic genetics applies molecular biology techniques to analyse DNA from biological samples for legal investigations. It encompasses the understanding of
Topic Synopsis
Forensic genetics applies molecular biology techniques to analyse DNA from biological samples for legal investigations. It encompasses the understanding of DNA structure and replication to ensure reliable extraction, amplification via PCR, and profiling using STR markers. The resulting DNA profiles are interpreted within the context of population genetics to provide evidential weight in criminal cases.
Key Concepts & Core Principles
- Locard's Exchange Principle: Every contact leaves a trace. This fundamental concept underpins forensic science, meaning that a perpetrator will always bring something to and take something from a crime scene, such as fibres, hair, or soil.
- Chain of Custody: The documented chronological record of evidence handling from collection to court presentation. Any break in the chain can render evidence inadmissible, so meticulous documentation is critical.
- Types of Evidence: Distinguish between direct evidence (e.g., eyewitness testimony) and circumstantial evidence (e.g., fingerprints). Also understand class evidence (e.g., blood type) vs. individual evidence (e.g., DNA profile) and their probative value.
- Analytical Techniques: Master key methods like thin-layer chromatography (TLC) for separating inks, gas chromatography-mass spectrometry (GC-MS) for identifying drugs, and UV-visible spectroscopy for quantifying substances. Know their principles, applications, and limitations.
- Crime Scene Processing: The systematic approach to securing, documenting, and collecting evidence. Steps include establishing a perimeter, photographing the scene, sketching, searching (e.g., grid, spiral methods), and packaging evidence to avoid contamination.
Exam Tips & Revision Strategies
- Always link the theoretical knowledge of DNA replication to practical forensic techniques; for example, explain how PCR mimics in vivo replication.
- Use precise terminology such as 'locus', 'allele', 'homozygous', and 'heterozygous' when describing DNA profiles to show command of the subject.
- Structure forensic genetics reports with clear sections: sample collection, extraction, quantification, amplification, analysis, and statistical interpretation.
- Practice calculating match probabilities using Hardy–Weinberg equilibrium and explain the assumptions and limitations of such calculations.
- Evaluate the reliability of DNA evidence by discussing potential issues like secondary transfer, partial profiles, and the prosecutor's fallacy.
- In written assignments, always link theoretical knowledge to practical forensic scenarios—for example, explain how understanding DNA replication aids in designing PCR primers.
- When tackling data interpretation questions, systematically annotate electropherograms to show allele calling and highlight any artifacts like stutter peaks.
- Prepare to discuss the evidential weight of DNA results by explaining the prosecutor's fallacy and correctly using likelihood ratios rather than stating 'the DNA matches the defendant'.
Common Misconceptions & Mistakes to Avoid
- Confusing the semi-conservative nature of DNA replication with a dispersive mechanism, leading to errors in understanding genetic continuity.
- Overlooking the impact of PCR inhibitors in forensic samples, resulting in failed amplification and false negatives.
- Misinterpreting low-level DNA mixtures as single-source profiles due to ignoring stochastic effects like allele drop-out.
- Failing to account for contamination risks at every stage, from crime scene collection to laboratory processing, which can compromise evidence integrity.
- Incorrectly assuming that a match at a few STR loci provides conclusive identification without considering population frequency statistics.
- Confusing DNA replication in vivo with PCR amplification, often misattributing in vivo enzymes (e.g., primase) to the PCR process.
Examiner Marking Points
- Award credit for correctly describing the steps of DNA replication and their relevance to maintaining genetic integrity during cell division.
- Award credit for accurately explaining the principles and procedures of DNA extraction, including cell lysis and purification, suited to different sample types.
- Award credit for meticulous demonstration of PCR setup, including primer design, thermal cycling conditions, and the role of Taq polymerase.
- Award credit for thorough analysis of STR profiles, including allele sizing, peak height interpretation, and recognition of artefacts like stutter bands.
- Award credit for applying statistical methods to calculate match probabilities and discuss the significance of DNA evidence in a forensic context.
- Award credit for demonstrating understanding of semi-conservative DNA replication, including roles of key enzymes (e.g., DNA polymerase, helicase) and the significance of origins of replication.
- Credit given for selecting and justifying an appropriate DNA extraction method (e.g., Chelex, organic, solid-phase) based on the nature of the biological sample (blood, hair, bone).
- Look for accurate description of PCR principles, including denaturation, annealing, and extension steps, and the function of Taq polymerase.