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