It's common knowledge that a mother and father's DNA help form their offspring's genetic make-up - but now scientists have thrown something else into the mix.
Researchers studying fruit flies found their progeny's size could be influenced by the length of a previous mate.
This may be because chemicals in the original male's seminal fluid can have a lingering effect, and scientists have even suggested that something similar could occur in humans.
The theory is known as "telegony" and was first proposed by Aristotle in ancient Greece. It was one of the reasons kings were banned from marrying divorcees.
The theory was discredited thanks to the advent of genetics. But now, writing in the journal Ecology and Evolution, scientists have suggested flings can influence the offspring of future mates.