An extremely rare set of semi-identical twins were identified by doctors in Australia in a groundbreaking discovery that was made during pregnancy, experts revealed this week.
The Brisbane twins, a boy and a girl, are only the second set of semi-identical, or sesquizygotic, twins to be identified in the world and the first to be identified in utero, according to a paper published in The New England Journal of Medicine (NEJM) Thursday.
What sets the twins, now 4, apart is that they are identical on their mother's side and share her full DNA but only share a proportion of their father's DNA.
"It is likely the mother's egg was fertilized simultaneously by two of the father's sperm before dividing," said Professor Fisk, who led the fetal medicine team that cared for the mother and twins while based at Royal Brisbane and Women's Hospital in 2014, in a statement. "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."
The first case of sesquizygotic twins was identified in 2007. At the time, geneticist Vivienne Souter, of the Banner Good Samaritan Medical Center in Phoenix, Arizona explained that "their similarity is somewhere between identical and fraternal twins," according to Nature. "It makes me wonder whether the current classification of twins is an oversimplification."
Commenting on the latest set of identical twins, Fisk noted it was an exceptional case.
"While doctors may keep this in mind in apparently identical twins, its rarity means there is no case for routine genetic testing." he said.
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