The twins share 100% of their mother's DNA, but only 78% of their father's DNA.
We know about identical and fraternal twins - the former being the Olsen twins, and the latter being confusing when you meet them. The twins, born in Brisbane, Australia, in 2014, were the first of their kind to be identified during pregnancy. Her ultrasounds at six weeks indicated she was expecting identical twins, but the 14-week scan "showed the twins were male and female, which is not possible for identical twins", Fisk said.
"It is likely the mother's egg was fertilised simultaneously by two of the father's sperm before dividing", Fisk said in the statement.
Nearly all twins are either fraternal - in which two eggs and two sperm create two separate embryos, or identical - in which one embryo is fertilised by one sperm and splits into two embryos.
"We think it's because two separate sperm fertilised a single egg".
As a result, the fertilised egg contains three sets of DNA chromosomes - two from the father and one from the mother.
"In the case of the Brisbane sesquizygotic twins, the fertilized egg appears to have equally divided up the three sets of chromosomes into groups of cells which then split into two, creating the twins".
The twins are what's known as "sesquizygotic", and their genetic makeup lies somewhere between fraternal and identical.
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For only the second time in history, semi-identical twins have been born, and this is the first time that they were identified during pregnancy.
Fetal medicine specialist and Deputy Vice-Chancellor (Research) at UNSW, Professor Nicholas Fisk, and QUT clinical geneticist and Diagnostics Genomics course coordinator Dr Michael Gabbett, identified the twins in the paper "Molecular Support for Heterogonesis Resulting in Sesquizygotic Twinning", published today in The New England Journal of Medicine.
Because of the odd combination of DNA picked up from the two sperm, doctors have been concerned that the twins might be vulnerable to cancer of the reproductive organs.
Initially, the mother of the twins described in the case report thought that she was pregnant with identical twins, based on an ultrasound early in her pregnancy. That just the first reported instance, meaning there certainly could be more cases out there. A resulting chromosomal investigation found the twins to share 100% of their mother's DNA but only half of their father's. The unusual genetic makeup of the twins was only discovered because one of them had ambiguous genitalia.
To see if the phenomenon might be more common than doctors believed, the Gabbett team examined an global database of 968 fraternal twins and their parents.
In 2006, twins in the U.S. were identified as semi-identical twins when they were examined as infants for another medical condition.
Though we may not see another set of semi-identical twins like this any time soon, Doctor Gabbett says that it's good to acknowledge incredible cases like that of these Queensland twins. "Its rarity means there is no case for routine genetic testing".