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As the name suggests, semi-identical twins (also called sesquizygotic twins) share some – but not all – of their parents' DNA. More precise, they share 100 percent of their mom's DNA but only a portion of their dad's.
That's different than fully identical twins, who share all the same DNA from their parents. And it's different than fraternal twins, who share the same womb but are no more similar than two siblings.
A doctor in Brisbane discovered the existence of semi-identical twins after a routine, 14-week ultrasound of a pregnant woman revealed that she was carrying a boy and a girl attached to the same placenta. Normally, sharing of the placenta is a hallmark of identical twins. But because identical twins are just that – identical – they can only be the same sex.
Puzzled, the researchers analyzed the babies' genes while they were still in the womb. What they found astonished them: The babies shared almost 90 percent of their DNA but were not completely identical.
How did this happen? The researchers aren't sure. But, as they wrote in the New England Journal of Medicine, they believe the mom's egg was fertilized by two separate sperm, and then the egg divided. Normally, this would result in an odd number of chromosomes, and the fetus would perish. But in this remarkable case, it appears the fertilized egg equally divided up the three sets of chromosomes into groups of cells that then split in two to form twins, the researchers said.
As it turns out, this isn't the first documented case of semi-identical twins. A doctor in the United States reported the first case in 2007.
Could there be more semi-identical twins out there? Dr. Mindy Christianson, an assistant professor at Johns Hopkins School of Medicine, told the New York Times that there may be others who haven't been identified because they're the same sex.
That said, a review of worldwide genetic data on almost 1,000 fraternal twins did not uncover any more that were semi-identical, the researchers reported. Since the type is so rare, there are unlikely to be any new genetic testing requirements of twins as a result of this discovery.
And the semi-identical Australian siblings? They're now 4 years old, healthy, and thriving.
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