The only other known case of semi-identical twins was two born in the United States in 2007. They are the first to have been observed in utero (in the womb).
"It is likely the mother's egg was fertilized simultaneously by two of the father's sperm before dividing", said Dr. Nicholas Fisk, a professor and maternal-fetal medicine specialist from the University of New South Wales in Sydney.
Fisk cautioned that the rarity of semi-identical twins meant there was no need for "routine genetic testing".
The identity of the twins has yet to be revealed.
However, they eventually realised that the twins were different sexes - meaning that it would have been impossible for them to be identical.
Franternal twins - those that aren't identical - share half of their DNA, whereas identical twins share all of their DNA.
"Some of the cells contain the chromosomes from the first sperm while the remaining cells contain chromosomes from the second sperm, resulting in the twins sharing only a proportion rather 100 per cent of the same paternal DNA".
The twins, born in Brisbane, Australia, in 2014, were the first of their kind to be identified during pregnancy. The other two packages each contained the same set of chromosomes from the mother, as well as genetic material from one of the two sperm, giving rise to either XX (female) or XY (male) cells.
The boy and girl, who are now four years old, are only the second known set of semi-identical twins in the world and the first to be identified by doctors during pregnancy.
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Fraternal twins happen when two different sperm cells fertilize two different eggs, creating two different zygotes, both of which end up implanted in the uterus as developing embryos. "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.
"We know this is an exceptional case of semi-identical twins", Fisk said.
In this case, it appears that the fertilized egg equally divided the three sets of chromosomes into groups of cells that split into two, creating twins, according to clinical geneticist Michael Gabbett, from Queensland University of Technology. The pair of twins, from the same egg, shared the same placenta and grew in the same amniotic sac in their mother's womb - which is only ever possible with identical twins.
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.
To see if the phenomenon might be more common than doctors believed, the Gabbett team examined an global database of 968 fraternal twins and their parents.
The authors also performed genetic tests of 968 sets of other twins presumed to be fraternal to see if any were really semi-identical.
The girl developed a blood clot in her arm that the doctors think is unrelated to sesquizygosity, Gabbett adds.
'Otherwise, ' Dr Gabbett said, 'the two twins are handsome kids, well and healthy'.