The second set of semi-identical twins in the world have been discovered in Queensland. They are also the first to be identified during pregnancy.
Two Queensland twins have made history after becoming the first pair of semi-identical twins in the world to be identified during pregnancy.
Twins of their kind do not usually survive as embryos, making the now four-year-old siblings just the second set of semi-identical twins in the world.
Deputy Vice Chancellor for Research at the University of New South Wales, Professor Nicholas Fisk made the world-first discovery in 2014.
Professor Fisk said he started looking into the twins' case after coming to the seemingly impossible realisation they were identical and of different sex.
“The issue here was they were of different sex, so when we did DNA studies it showed they weren't identical, but they weren't non-identical, either - they were really halfway in-between,” he told SBS News .
Doctors and scientists then launched into a series of tests, discovering the twins were identical in DNA inherited from their mother, just like identical twins, but had different DNA inherited from their father, just like non-identical twins, or even regular siblings.
Now, after going through thousands of other cases of twins around the world, researchers have confirmed the twins were the first of their kind to be identified in pregnancy.
“We think it's because two separate sperm fertilised a single egg,” Professor Fisk said.
“Now, that shouldn't happen - when a sperm enters an egg, the egg locks down to prevent one of the millions of other sperm getting in there as well. If it does get in there, you should have three, rather than two sets of chromosomes, which means the embryo is usually not viable.”
But these two were viable.
They defied the odds, survived, and have now achieved each and every one of their major developmental milestones.
Queensland University of Technology clinical geneticist Doctor Michael Gabbett, who did extensive testing on the twins after Professor Fisk's discovery, said the twins could easily pass for identical in appearance.
"I recently saw them late last year again and they look like their parents as any children do, I couldn't say if they look more like their mother or their father, but it's quite striking how similar they look to each other,” he told SBS News.
“I think if you dressed one up as a boy or the boy up as a girl, you could pass them off as identical twins easily."
The only other known case of semi-identical twins is in the United States, where twins who were also both identical and fraternal were born in 2007.
However, they were not identified as semi-identical until after their birth.
While the Queensland discovery is likely to have little bearing on everyday medicine, Doctor Gabbett said it stands for something equally as important.
"It reminds us that we should always keep an open mind in medicine,” he said.
“We traditionally classify twins as being identical or non-identical, and while that classification works, sometimes things don't always appear as they seem.
“If there is the suspicion that something isn't fitting into the categorisation that we normally use, then we have to think outside the box."