Genes of mother's ex-partner show up in young flies. As for humans …

What if your genetic destiny wasn't written solely by your two biological parents, but also by your mother's first sexual partner? That is the case for neriid flies, which seem to develop differently based on the traits of their mothers' previous partners.

Researchers had observed that these flies seemed to inherit their stature in an unusual way: if flies were fed well and fattened up, scientists observed, their offspring would be bigger. In other words, fly daddies seemed to be passing along their nurtured size as a "natured" trait.

In a study published in Ecology Letters, researchers set out to prove that it was something in the flies' semen - not their genes - that caused the size change.

"The entire ejaculate has a whole bunch of other things in it; only five per cent is the sperm itself," lead study author and University of New South Wales post-doctoral researcher Angela Crean told Popular Science.

"The sperm is what fertilises the egg, but you have all these sugars and proteins and fluid that carry that sperm," she said. And it's possible that some of those molecules are designed to affect female reproduction.

To test the theory, Crean and her colleagues produced large and small male flies by way of feeding, then mated them with immature females. Later, these females were mated again - and this time, fully mature, they produced offspring.