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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Michael J. Spence, Ph.D.
SUSPECTThe Locard Exchange Principal“Every contact leaves a trace.”
The most common sources of DNA:
*All nucleated cells*
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-vehiclesAdditional Sources of DNA Often Examined by Crime Labs
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
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
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
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
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?
Compare each DNA profile
from evidence to profiles
collected from individuals