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 Animation demonstrating how DNA mixtures at a crime scene can occur.

August 5, 2020
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          Transcript
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.
    
  
Disclaimer:
              Opinions or points of view expressed in these recordings represent those of the speakers and do not necessarily represent the official position or policies of the U.S. Department of Justice. Any commercial products and manufacturers discussed in these recordings are presented for informational purposes only and do not constitute product approval or endorsement by the U.S. Department of Justice.
  
Date Published: August 5, 2020
    
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