A pair of Australian twins are among the rarest humans ever documented, a study released Thursday reveals.
The Brisbane siblings — a boy and girl — have been classified as just the second reported case of sesquizygotic, or semi-identical, twins in the world, and the first to be identified while still in their mother’s womb, according to the paper, published in the New England Journal of Medicine.
Sesquizygotic twins, genetically, fall somewhere in between fraternal and identical twins.
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.
Early in the second trimester, it became clear that there was something unusual about them.
“They are 100 percent identical on the mother’s side and 78 percent identical [on] the father’s side, so this averages out to being 89 percent identical,” Gabbet told Gizmodo.
It’s believed that the twins — now four years old — were formed when one egg was fertilized simultaneously by two sperm, according to Fisk.
“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 in a statement. “However, an ultrasound at 14 weeks showed the twins were male and female, which is not possible for identical twins.”
Fisk immediately called Gabbett about the surprising results.
“I was a bit cheeky and accused Professor Fisk of having inadequate ultrasound skills, but turns out they were perfectly good,” Gabbett told the Australian Broadcasting Corporation. “It was a very unusual genetic event.”
It’s possible for double fertilization to occur, but those embryos don’t typically survive, Gabbett told the outlet.
The family declined to be identified, but the twins are doing well and reaching all their development milestones, according to the doctor.
“They look very similar,” he told the outlet. “They’re very cute and they’re doing very well.”
The researchers examined genetic data from 968 other fraternal twins and their parents — but did not find a set of sesquizygotic twins.
“It was a very unusual genetic event,” Fisk told the ABC, noting that there has only been one other reported case — in the U.S. in 2007.