Chapter 7

Forensic Issues: Degraded DNA, PCR Inhibition, Contamination, and Mixed

Samples

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D19

AMEL

D3

D8 VWA

TH01

D21FGA D16 D18

D2

D19AMEL D3

D8 VWA

TH01

D21

FGA

D16

D18 D2

15 years old (room temp storage)

6 years old (-20 oC storage)

Results with SGM Plus STR kit (Applied Biosystems)

“Decay curve” of degraded DNA

Degraded DNA Results

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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%<15%Stutter region

>70%>70%

100%

Heterozygous peak region

85%

MIXTURE REGION

MIXTURE REGION

9%

Typical Single Source STR Heterozygote Profile

Example Mixed Sample STR Profile

Higher than typical stutter product (>15%)

100%

<15%<15%

>70%>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 InterpretationIdentify 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

VWAFGAD3S1358 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 FGA D5S818

Chromosome 21 Chromosome 4 Chromosome 5

TPOX

Chromosome 2

CSF1PO

Chromosome 5 Chromosome 16

D16S539

©2002 Academic Press