Chapter 7 Forensic Issues: Degraded DNA, PCR Inhibition, Contamination, and Mixed Samples ©2002 Academic Press

D19 AMEL D3 D8 VWA TH01 D21 FGAD16D18 D2 D19 AMEL D3 D8VWA TH01 D21 FGA D16 D18 D2 15 years old (room temp storage) 6 years old (-20 o C storage) Results with SGM Plus STR kit (Applied Biosystems) “Decay curve” of degraded DNA Degraded DNA Results ©2002 Academic Press

Crime Scene Sample Perpetrator (MALE) Victim (FEMALE) Identifying the Perpetrator from a Crime Scene Sample Mixture Can Be Challenging See Butler, Forensic DNA Typing: Biology and Technology Behind STR Markers, Academic Press, 2001 Problem of Mixtures

Example of Single Source vs. Mixture DNA Profiles <15% Stutter region >70% 100% Heterozygous peak region 85% MIXTURE REGION 9% Typical Single Source STR Heterozygote Profile Example Mixed Sample STR Profile Higher than typical stutter product (>15%) 100% <15% >70% 60% 10% 25% Wrong side of allele to be typical stutter product Smaller peak area than normally seen with heterozygote partner alleles(<70%) ©2002 Academic Press

Steps in Mixture Interpretation Identify the Presence of a Mixture Consider All Possible Genotype Combinations Estimate the Relative Ratio of the Individuals Contributing to the Mixture Identify the Number of Potential Contributors Designate Allele Peaks Compare Reference Samples Step #1 Step #2 Step #3 Step #4 Step #5 Step #6 ©2002 Academic Press

D5S818 D13S317 D7S820 D8S1179 D21S11 D18S51 Amel VWA FGA D3S1358 blue panel green panel yellow panel DNA Size (bp) Relative Fluorescence Units Sample Mixture Example ©2002 AP, Elsevier

Abnormal STR Peak Heights from K562 Cell Line D21S11 FGAD5S818 Chromosome 21Chromosome 4Chromosome 5 TPOX Chromosome 2 CSF1PO Chromosome 5 Chromosome 16 D16S539 ©2002 Academic Press