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Understanding DNA Transfer Events

Understanding DNA Transfer Events. Michael J. Spence, Ph.D. WHAT IS DNA ?. D EOXYRIBO N UCLEIC A CID DNA is the genetic blueprint that encodes your physical appearance You receive ½ of your DNA from your mother & ½ from your father. CRIME SCENE. VICTIM . SUSPECT.

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Understanding DNA Transfer Events

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  1. Understanding DNA Transfer Events Michael J. Spence, Ph.D.

  2. WHAT IS DNA? • DEOXYRIBONUCLEIC ACID • DNA is the genetic blueprint that encodes your physical appearance • You receive ½ of your DNA from your mother & ½ from your father.

  3. CRIME SCENE VICTIM SUSPECT The Locard Exchange Principal“Every contact leaves a trace.” 1910

  4. Nucleated Cells What can DNA tell usabout a crime? Determine the DNA profile Compare each DNA profile from evidence to profiles collected from individuals

  5. Blood Semen Hair Root Saliva Bone The most common sources of DNA: *All nucleated cells* Extraction

  6. Epithelial (skin) cells deposited or transferred Ski masks, Hat bands, gloves, other clothing Towels, washcloths, sheets, and bedding Doorknobs, cabinet, and faucet handles Combs, toothbrushes, personal grooming items Rings, earrings, and other jewelry Drink containers, cigarette butts, chewing gum Stamps & Envelopes Steering wheel, gear shift, door handles-vehicles Additional Sources of DNA Often Examined by Crime Labs

  7. The Locard Exchange Principal. Frequent transfer of viruses-colds/flus. Radioactive DNA contamination. Contamination in police labs Are DNA Transfer Events Fact or Speculation?

  8. Intimate contact, kissing and/or sexual activity, hugs, handshakes, etc. Clothing/bedding/towels used by 1st person, then shared by a 2nd person. Sharing cigarettes/drink containers, etc. Skin cells transferred during handling of door knobs/handles, phones, pens, pencils, computer keyboards, car keys, steering wheels, sunglasses, stairwell railings, vending machines, etc. Transfer events-cells and DNA

  9. The higher the 'liquid' content (blood, sweat, tears, saliva), the more readily cells and DNA will be transferred. Coughing, sneezing directly onto an item or transferred via handkerchiefs, facial tissue. Transfer from a used pillow case (sweat, saliva, skin cells) onto a firearm that has been placed under the pillow. Transfer events: cells and DNA

  10. The average adult human body is covered with over 100 BILLION skin cells (this is 6 pounds of cells) An individual will shed roughly 600 skin cells, …..PERSECOND. A person might drool 5 ml (1 or 2 ketchup packets?) of saliva onto a pillow during each night’s sleep. This leaves 40,000,000 salivary epithelial cells deposited onto that pillow case. Important facts: skin & saliva cells

  11. A Forensic Biologist needs only ONE nanogram of DNA to obtain a FULL DNA profile. ONE nanogram of DNA can be extracted from as few as 200 skin or saliva cells. When a person sleeps on a pillow case, night after night, and deposits billions of skin and/or saliva cells onto that surface, the transfer rate of cells/DNA from the pillow to a firearm is UNKNOWN. Skin & saliva cells: continued

  12. How much IS a nanogram? How much is 1 Gram? One packet of artificial sweetener How much is 1 mg? 1/1000 of a Gram How much is 1 mg? 1/1000 of a mg How much is 1 ng? 1/1000 of a mg or 1 Billionth of 1Gram How many cells to get 1 ng of DNA? Approximately 200 Compare each DNA profile from evidence to profiles collected from individuals Determine the DNA profile

  13. How? No When? No Who? Yes Substrate Controls? What DNA can and cannot tell us:

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