An illustration of two sperm fertilising one egg - this results in semi-identical twins.
A set of Queensland twins have been identified as just the second semi-identical twins in the world and the first to be discovered during pregnancy.
The boy and girl share 100 per cent of their mother's genes but share only a proportion of their father's DNA.
The twins were born four years ago but it's taken researchers several years to confirm if they were in fact sesquizygotic, or semi-identical.
A paper about them was published on Thursday in The New England Journal of Medicine.
One of the authors, fetal medicine specialist and UNSW deputy vice-chancellor Professor Nicholas Fisk, was working in Brisbane when he came across the then-pregnant mother.
"The mother's ultrasound at six weeks showed a single placenta and positioning of amniotic sacs that indicated she was expecting identical twins," he said.
"However, an ultrasound at 14 weeks showed the twins were male and female, which is not possible for identical twins."
Prof Fisk said after extensive genetic testing to rule out chromosomal abnormalities, they concluded the most likely scenario was the mother's egg was simultaneously fertilised by two of the father's sperm before dividing.
"We then started to wonder if over the decades there had been some twins who had been told they were non-identical but in fact could also be three-quarters identical," he said.
Prof Fisk said they examined genetic data from 968 fraternal twins and their parents.
"However, we found no other sesquizgotic twins in these data, nor any case of semi-identical twins in large global twin studies," he said.
Prof Fisk said the twins, who are healthy and developing well, were a rarity.
The only other reported case of sesquizygotic twins occurred in the US in 2007, when the boy and girl were infants.
© AAP 2019
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