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Forensic Biology by Richard Li

Chapter 18: Autosomal STR Profiling. Forensic Biology by Richard Li. Basics. Regions of the human DNA with an array of tandem repeats Length of repeat can be 2-6 bp long Microsatellites or Simple Sequence Repeats Most common STR loci: 100-500 bp Advantages of STR loci:

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Forensic Biology by Richard Li

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  1. Chapter 18: Autosomal STR Profiling Forensic Biologyby Richard Li

  2. Basics • Regions of the human DNA with an array of tandem repeats • Length of repeat can be 2-6 bp long • Microsatellites or Simple Sequence Repeats • Most common STR loci: 100-500 bp • Advantages of STR loci: • Applicable for PCR amplification • Better for degraded DNA • Preferential amplification is reduced • Better electrophoretic resolution • Multiplexing amplification • Mixed profiling Forensic Biology by Richard Li

  3. Characteristics of STR Loci • More than 10⁵ STRs in human genome • Core repeat region & Flanking regions • Repeat Unit Length • Number of nucleotides in a single unit of tandem repeat • Dimeric, Trimeric, Tetrameric, Pentameric, Hexameric • Core repeat Sequences • Simple • Complex Forensic Biology by Richard Li

  4. STR Commonly Used • Population Match Probability • STR loci should be highly variable • Loci should not be linked to each other • STR multiplex system in U.K.- 4 loci • 1995- first national database est. in U.K. • 6 STR loci + amelogenin (4 loci added later) • 1997- first database in U.S. • CODIS • FBI • 13 core loci + amelogenin Forensic Biology by Richard Li

  5. STR Commonly Used • European Standard Set (ESS) • 1998 • 7 loci • Other: • German database has 8 loci Forensic Biology by Richard Li

  6. Forensic STR Analysis • Loci are amplified using fluorescent dye-labeled primers • Separated using electrophoresis and detected • Results in an Electropherogram • Profile of peaks • Size of STR fragment determined by an internal size standard • Standard curve est. Forensic Biology by Richard Li

  7. Factors Affecting Genotyping Results • Mutations at STR core repeat regions • Chromosomal and Gene Duplications • Point mutations • Amplification artifacts • Allelic dropout • Electrophoretic artifacts • Pull-up • Spikes Forensic Biology by Richard Li

  8. Genotyping of Challenging Forensic Samples • Degraded DNA • MiniSTR multiplex kits • Low-copy Number DNA (LCN) • < 100 pg of DNA • Mixtures • Sexual assault cases • Mixture interpretation Forensic Biology by Richard Li

  9. Interpretation of Results • SWGDAM & DNA Commission of the ISFG: • Inclusion (Match) • Exclusion • Inconclusive Forensic Biology by Richard Li

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