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.
Professor Russell Bonduriansky, of the University of New South Wales in Australia, said: "Males contribute DNA to fertilise an egg, but we believe there is something more complex going on."
His researchers studied the offspring of female flies that mated with two males of varying sizes. They found the size of the young could be determined by the size of the first male rather than the male that fathered the offspring.
This could be because seminal fluid is chemically complex, with messenger molecules called RNA floating in the liquid. Professor Bonduriansky said: "There is evidence these RNAs can play a role in embryonic development. But the jury is still out on exactly what effects these molecules have."
- Daily Mail