Chapter 18 Autosomal STR Profiling Forensic Biology by

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: Applicable for PCR amplification Better for degraded DNA Preferential amplification is reduced Better electrophoretic resolution Multiplexing amplification Mixed profiling Forensic Biology by Richard Li 2

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 3

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 4

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

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 6

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 7

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

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