0:01 Commonly, evidence found at a crime scene will not have just one person's DNA on it,
0:06 but will instead have a mixture of DNA from more than one contributor.
0:10 These mixtures are most commonly encountered in sexual assault evidence but frequently occur in
0:15 other types of cases as well. This animation will show step-by-step a sample murder case
0:21 that demonstrates just one example of how multiple individuals DNA can end up on a piece of evidence.
0:29 Event One: the homicide victim borrows her boyfriend's sweaty, dirty shirt to wear while
0:35 cleaning the house residual traces of his DNA remain in the shirt from when he last wore it.
0:44 Event Two: while working, the victim wiped sweat from her brow with the sleeve of her shirt
0:50 transferring some of her own DNA in the process.
0:57 At this point, multiple people have left their genetic material on the shirt,
1:01 causing a mixture of DNA.
1:06 Event Three: a stranger breaks into the house and encounters the victim,
1:10 who grabs a kitchen knife to defend herself; through an ensuing struggle the victim cuts
1:15 the perpetrator some of whose blood lands on the sleeve of the shirt.
1:25 Event Four: the intruder succeeds in wrestling the knife away from the victim and stabs her with it;
1:31 the area surrounding her wound will have too much of her own blood to provide a useful
1:35 sample of anything else, so investigators will have to test other areas of the shirt.
1:44 Event Five: the victim's roommate arrives home and startles the perpetrator.
1:49 She too struggles with the intruder who is still holding the knife; the roommate sustains defensive
1:55 wounds to the hands and forearms but is not killed. The perpetrator ends up fleeing the scene.
2:04 Event Six: after the perpetrator has left, the roommate goes over to check on the victim.
2:10 Upon placing her lacerated hand on the victim's
2:12 arm the roommate transfers her own blood to the shirt sleeve.
2:21 When the shirt arrives at the lab and is analyzed by the technician, four different sets of DNA will
2:26 show up together on the sleeve. More clues will be necessary in order for the lab technician to
2:32 be able to sort out the combined DNA and determine who the four contributors were.
