DOCTORS say they have discovered the world’s second known case of semi-identical twins born from one egg and two sperm.
The now four-year-old boy and girl, from Brisbane, Australia, are identical on their mother’s side but only share a proportion of their father’s DNA.
Experts say the phenomenon, known as semi-identical or sesquizygotic twins, is extremely rare and typically these embryos don’t survive.
The case, reported on in The New England Journal of Medicine, is the first to have been identified during pregnancy.
Doctors said the first-time mum, who was 28 at the time, had conceived naturally.
Prof Nicholas Fisk, who led the team that cared for the mum and twins at the Royal Brisbane and Women’s Hospital in 2014, said the discovery was made through a routine scan.
He said: “The mother’s ultrasound at six weeks showed a single placenta and positioning of amniotic sacs that indicated she was expecting identical twins.
“However, an ultrasound at 14 weeks showed the twins were male and female, which is not possible for identical twins.”
Prof Fisk added: “It is likely the mother’s egg was fertilised simultaneously by two of the father’s sperm before dividing.”
Genetically, they are somewhere between fraternal and identical twins.
Identical twins result when cells from a single egg fertilised by a single sperm divide into two – so they are the same gender and share identical DNA.
Fraternal twins occur when each twin develops from a separate egg and the egg is fertilised by its own sperm.
Dr Michael Gabbett, of Queensland University of Technology who worked alongside Prof Fisk, explained that if one egg is fertilised by two sperm it results in three sets of chromosomes, one from the mother and two from the father.
He said: “Three sets of chromosomes are typically incompatible with life and embryos do not usually survive.
“In the case of the Brisbane sesquizygotic twins, the fertilised egg appears to have equally divided up the three sets of chromosomes into groups of cells which then split into two, creating the twins.
“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.”
FIRST RARE CASE
The first sesquizygotic twins were reported in the United States in 2007.
They came to doctors’ attention in infancy after one was identified with ambiguous genitalia.
On investigation of mixed chromosomes, doctors found the boy and girl were identical on their mother’s side, but shared around half of their paternal DNA.
Prof Fisk said an analysis of worldwide twin databases pointed to just how rare sesquizygotic twins are.
He said: “We at first questioned whether there were perhaps other cases which had been wrongly classified or not reported, so examined genetic data from 968 fraternal twins and their parents.
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“However we found no other sesquizygotic twins in these data, nor any case of semi-identical twins in large global twin studies.”
Prof Fisk added: “We know this is an exceptional case of semi-identical twins.
“While doctors may keep this in mind in apparently identical twins, its rarity means there is no case for routine genetic testing.”
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