Collecting DNA Evidence

Most common – blood and semen If exposed to environment – risk of DNA breakdown and contamination Keep bio evidence on original surface, if possible If not, swab surface for evidence, plus comparative samples

If exposed to envir., cell can open and degrade DNA – hard to gain good evidence RFLP Analysis – sensitive to degradation, but still may give useful DNA profile Likely to degrade if exposed to certain enzymes or microorganisms Warm, moist environs increase chance of exposure

Need to remove moisture – dry and freeze sample ASAP Take item with bio evid (or swab) directly to lab

Collecting Biological Evidence at a Crime Scene No bare hands; no sneezing or coughing near evidence Clean gloves for each piece of evidence Package separately Air dry objects with wet evidence Dry evidence in paper bag or envelope

Cotton swab and water if cannot remove piece of evidence Reference samples – known for comparison Taken from suspects, victims, family members

Presumptive Tests At crime scene For blood – - Luminol – sprayed on large area; needs darkness to see glow * picks up blood even if diluted 10 million X * doesn’t destroy DNA

50% + of DNA cases involve sexual assault For semen – - Chemicals and fluorescent dyes to see seminal fluid - look for acid phosphatase - Kits test for prostate-specific antigen (PSA) - Microscope to detect sperm

For saliva – test for amylase (breaks down starch) - Phadebas reagent – starch w/ dye; when reagent & amylase mix, dye is released - Starch iodine test – turns blue

Mitochondrial Analysis Circular DNA in mitochondria (mtDNA) Only inherited from mother Many copies per cell Less degradation Not unique – same for all of a mother’s children Useful in missing persons and mass disasters

Mostly from teeth, hair, bones (femur) Extracted to separate from PCR inhibitors Can spare bones and teeth for physical exam Hair sample destroyed, so examined first, then extracted

Y-Chromosome Analysis Inherited father to son Can match a father, brother, or paternal relative Useful in sexual assault, missing persons, genealogy Not individual evidence, but useful if little DNA left