The discovery was actually made before the twins were born. A scan at the Royal Brisbane and Women's Hospital in 2014, early in the pregnancy, showed "a single placenta and positioning of amniotic sacs that indicated she was expecting identical twins," Nicholas Fisk, who was head of the mother's care team, explained in a report by Queensland University. "However, an ultrasound at 14 weeks showed the twins were male and female, which is not possible for identical twins."
Doctors determined that a single egg must have been fertilized by two sperm before it divided — resulting in the exceedingly rare sesquizygotic twins.
When one egg is fertilised by two sperm, it creates three chromosome sets: one from the mother and two from the father's two distinct sperm, noted Michael T. Gabbett, a Queensland University of Technology geneticist who was also part of the care team and an author of the brief published in the New England Journal of Medicine on the phenomenon.
Embryos with three sets of chromosomes, however, usually don't 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," Gabbett said in the QUT report.
Some cells contain chromosomes from the first sperm, others hold chromosomes from the second, so the twins will only share some of their paternal DNA — 78 percent in this case.
It is so unusual to encounter living semi-identical twins that the Australian researchers actually went back over genetic data from 968 fraternal twins and their parents to see if the phenomenon was simply being missed. But they found no other cases of sesquizygotic twins in that sample, nor in worldwide twin studies, Fisk explained. That makes the Brisbane twins only the second known pair in the world — and the first to be identified while in the womb.
The first pair of these twins were identified in the US in 2007, but only after they were born.