What is a likely source of DNA found on a toothbrush?

• A. DNA from toothpaste
• B. DNA that is synthesized chemically
• C. Cells shed from the mouth or saliva
• D. DNA from soil in the bathroom

Answer: (C)

The main idea here is that DNA on a personal item like a toothbrush usually comes from materials that come from the person’s own body during normal use. When you brush, the mouth’s lining sheds epithelial cells and saliva is present; both contain DNA from the person and from oral microbes. The toothbrush collects those cells and the DNA present in saliva, making human DNA from mouth cells the most likely source.

Toothpaste isn’t a DNA source for the person, as it isn’t made to include human DNA in meaningful amounts. DNA that’s chemically synthesized would have to be added deliberately, which isn’t typical for a bathroom toothbrush. DNA from soil or environmental sources could only appear if the toothbrush contacted soil, which isn’t the usual situation in a bathroom. So the most probable source is the cells shed from the mouth and the DNA in saliva.