The twins, who are identical on their mother's side but share only part of their father's DNA, are the first case of semi-identical, or sesquizygotic, twins identified in Australia.
Nearly all twins are either fraternal - in which two eggs and two sperm create two separate embryos, or identical - in which one embryo is fertilised by one sperm and splits into two embryos.
The semi-identical twin boy and girl born in Australia were found to have 100 per cent of their mother's DNA in common, but were only 78 per cent identical in the paternal DNA they carry.
Michael Gabbett, a geneticist at Queensland University of Technology, and one of his co-authors, Nicholas Fisk, an obstetrician and deputy vice-chancellor of research at the University of New South Wales, oversaw the twins' fetal care back in 2014 at the Royal Brisbane and Women's Hospital. But when the woman came in for a follow-up ultrasound at 14 weeks, it was discovered that she was carrying a boy and a girl-something that is impossible in identical twins. They were identical on their mother's side but shared about half of their father's DNA.
Two of the these had enough egg DNA and sperm DNA to make viable embryos.
"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. Fraternal twins occur when each twin develops from a separate egg and the egg is fertilized by its own sperm.
Now 4, the Brisbane twins, a boy and a girl, are the second set of semi-identical, or sesquizygotic, twins in the world, according to a press release about the study. "It's this odd place in between", chief author Dr. Michael Terrence Gabbett of Queensland University of Technology in Brisbane told Reuters Health in a telephone interview.
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This would normally result in a miscarriage, but in the case of the Australian twins, the fertilized egg formed three cells: one had DNA from the egg and the first sperm, the second had DNA from the mother and the second sperm, and the third had DNA from the two sperm.
The only other reported instance of so-called sesquizygotic twins was identified in 2007, brought to the attention of doctors because one had ambiguous genitalia.
The team analyzed genetic data from 968 fraternal twins as well as from other studies but found no other cases.
"While doctors may keep this in mind in apparently identical twins, its rarity means there is no case for routine genetic testing".
"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.
After birth, one twin experienced a blood clot that required the amputation of one arm, the authors wrote.
'Otherwise, ' Dr Gabbett said, 'the two twins are attractive kids, well and healthy'.