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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

Understanding DNA Transfer Events

Michael J. Spence, Ph.D.

what is dna
  • DNA is the genetic blueprint that encodes your physical appearance
  • You receive ½ of your DNA from your mother & ½ from your father.
what can dna tell us about a crime



What can DNA tell usabout a crime?

Determine the

DNA profile

Compare each DNA profile

from evidence to profiles

collected from individuals




Hair Root



The most common sources of DNA:

*All nucleated cells*


additional sources of dna often examined by crime labs
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
are dna transfer events fact or speculation
The Locard Exchange Principal.

Frequent transfer of viruses-colds/flus.

Radioactive DNA contamination.

Contamination in police labs

Are DNA Transfer Events Fact or Speculation?
transfer events cells and dna
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
transfer events cells and dna1
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
important facts skin saliva cells
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
skin saliva cells continued
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

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

what dna can and cannot tell us
How? No

When? No

Who? Yes

Substrate Controls?

What DNA can and cannot tell us: