Sydney scientists have shown for the first time that offspring can resemble their mother's previous sexual partner – in flies, at least.

The research team, led by evolutionary ecologist Angela Crean, propose that sperm from a previous partner can penetrate a developing egg, influencing its growth despite being sired by another male.

Dr Crean said her team were shocked when their experiments revealed they had discovered a new form of non-genetic inheritance.

"We did a lot of follow-up studies to check our results," she said.

First proposed in ancient Greece, the idea that offspring can inherit characteristics from their mother's previous mate – known as telegony – was discredited when scientists established more than a century ago that genes were the dominant way traits passed from parent to offspring.

"Before we discovered genetics it was widely believed that [telegony] occurred, and it was even spoken about by Darwin in The Origin of Species," Dr Crean, from the University of NSW, said.

"But once we figured out genetics, it didn't make sense under than mechanism, so it was just dismissed."

But more recently other forms of non-genetic inheritance have been observed, including work by Dr Crean which found a father's environment could influence the size of their offspring.

For instance, flies fed a nutrient-rich diet as maggots grew into bigger insects and then passed this condition onto their offspring.

Maggots fed a poor diet become smaller adults, as did their offspring.

"That's why we know it was not a genetic effect, because we manipulated the condition of the flies ourselves," she said.

To uncover how these traits were being passed between parent and offspring, Dr Crean and her collaborator Russell Bonduriansky took the research a step further, mating the small and large male flies with females and then studying their young.

They found that the size of the young was determined by the size of the first male the mother mated with, rather than the second male that sired the offspring. Their results are published in the journal Ecology Letters.

"Now we have to go down the very difficult path of trying to figure out how this happens," she said.

Dr Crean said it was likely something in the semen was influencing the growth of fly offspring. "But there are hundreds of different molecules in the semen, [so] it could be quite challenging to figure that out."

Dr Crean said this type of non-genetic inheritance had not been observed in other species, but there were clues from rodent studies that the phenomenon may be more widespread.