spdx-dna analysis and bloodstain pattern

• Forensic DNA Analysis


  Method for forensic scientist to determine who's the suspect in a crime scene
• Deoxyribonucleic Acid (DNA)


  Molecule found in nearly all cells (except anucleated cells: RBCs)<|>Known as a polymer (a molecule made up of repeating simpler units called monomers)<|>Located in 2 regions (nucleus & mitochondria)
• Nuclear DNA


  • Found in a geometric shape called a double helix
  • 2 strands (double-stranded) of nitrogenous bases which are complementary to each other
  • Made up of alternating sugar molecules (deoxyribose) & phosphates connected by phosphodiester band
  • Dangling off each sugar molecule is 1 of 4 bases or nucleotides (Adenine, Cytosine, Guanine, Thymine) connected by glycosidic linkage
  • Complementary bases or nitrogenous bases are connected by hydrogen bond
• Restriction Fragment Length Polymorphism (RFLP)


  1. DNA is extracted from biologic material & then severed into small fragments called using restriction enzymes (minisatellites)

  2. The length polymorphism present in these minisatellites is used to discriminate a population of people

  3. In forensic analysis, 4-6 of these highly polymorphic loci are analyzed
• Polymerase Chain Reaction (PCR)


  1. DNA added to PCR containing reaction mixture (master mix that compose primer, dNTPs, DNA polymerase, buffer)

  2. Heated to 95oC yields dsDNA denatures, bonds between the base pairs holding the strands together break, resulting in ssDNA

  3. Attach a short strand of synthetic DNA (called primers) to each of the separated strands, primers serve as starting point for the addition of new base to complete reproduction of each strand

  4. DNA polymerase catalyzes formation of phosphodiester bond to replicate DNA by extending primers, under the influence of Taq polymerase, single bases (nucleotides) are added to the primer
• Mitochondrial DNA (mtDNA)


  mtDNA is circular in shape<|>More numerous than genomic DNA (each cells has only 2 copies)<|>All male & female mtDNA comes from mother<|>mtDNA often shows a high degree of variation between unrelated people making it a powerful tool in forensic typing
• Bloodstain Pattern Analysis


  Analysis & interpretation of dispersion, shape characteristics, volume pattern, number, & relationship of bloodstains at a crime scene to reconstruct a process of events
• Bloodstain Terminologies


  • Passive Bloodstain: clots, drops, flows, & pooling
  • Transfer Bloodstain: wipes, swipes, pattern transfers, & general contact blood stains
  • Projected / Impact Bloodstains: spatters, splashes, cast-off stains, & arterial spurts or gushes, other patterns: fly spots voids, skeletonized stains
• Wipe Stain


  Created when an object moves through a pre-existing bloodstain (ex: clean rag moved through a pool of blood)
• Swipe Stain


  Transfer of blood onto a target by a moving object is itself bloodstained (ex: bloodied rag being moved through unstained floor)
• Spatter


  Technical term describing stains resulting from blood hitting a target
• Forward Spatter


  Blood droplets projected away from item
• Back Spatter


  Blood droplet being projected toward item
• Cast-Off Stain


  Blood being flung or projected from a bloody object in motion or one that stops suddenly
• Arterial Spurts / Gushes


  Occurs when an artery is breached while heart is pumping; produces zigzag, up & down pattern
• Voids


  Indicator that some secondary object came between a blood spatter & the final target; leaves outline or "shadow" on final target
• Point-of-Origin


  Whenever the direction of a bloodstain can be determined, it can be expected to have originated at a point somewhere along that line
• Documentation


  1. Document the entire scene as discovered, including "establishing" photographs

  2. Photograph patter transfers, pools, & other fragile patterns first

  3. Document patterns with "establishing" photographs that show the pattern's relationship to landmarks or other items of evidence

  4. Take macro & close-up photographs; include a scale in every photograph

  5. When reconstructing point of origin, document individual stains used in the reconstruction