The mystery behind the world's second ever 'semi-identical' twins

Normally twins are either "fraternal" meaning they grew from two separate eggs in the same womb, or "identical" meaning a fertilised egg splits into two viable fetuses.

The extremely rare third category is referred to by scientists as "sesquizygotic" twins, and is a result of two sperm fertilising the same egg.

Fetal medicine specialist Nicholas Fisk from the University of NSW said the third category was "as rare as hen’s teeth".

“The moment a sperm gets to an egg it triggers a big reaction which blocks any other sperm from getting through,” Professor Fisk said.

“So that usually stops it, and if a second sperm does get through it normally results in a condition called triploidy, where instead of having two copies of every chromosome, you have three copies, and it’s usually lethal.”

In this case however, it was anything but, with the boy and girl born happy and healthy.

Each of them has a full copy of their mother's genes, but only around half of their father's, meaning they share around 75 per cent of their DNA.

The twins were born in 2014, but the paper was not published until Thursday in The New England Journal of Medicine, which professor Fisk said was a sign of how long it took them to get to the bottom of the genetic mystery.

“It’s taken a long time to dot the Is and cross the Ts on all the genetic studies, but we didn't have a clue at the time how it had occurred,” he said.

The paper’s co-author, Queensland University of Technology clinical geneticist Michael Gabbett, said the only other case of semi-identical twins was a US pair, also a boy and a girl, found in 2007.

“DNA technology has improved a lot since then, so we have the tools available to say (the Australian case) is unequivocally halfway between fraternal and identical,” Dr Gabbett said.

“It seems to be extraordinarily rare. We also looked at twin repositories, lots of data from all different sets of twins, and we couldn’t find any other cases.”

While twins are becoming slightly more common due to the increased use of IVF for conception, the researchers noted neither the Australian nor US cases were IVF.

The twins will be monitored as they age, both due to their unusual genetic makeup and also because that very makeup makes them a risk for a rare type of cancer called gonadoblastoma.

“But in terms of their physical development they’ve hit all their milestones,” Dr Gabbett said.

"They’re two very cute, happy kids who happen to look much more similar than a regular brother and sister do."

Sesquizygotic twins have been observed in cows, but it was previously thought to be almost impossible for human fetuses to form that way.

“It just goes to show that biology is always surprising, and when you think you know it all, you don’t,” he said.

“There are these exceptional cases in biology which always cause us to stop and think about our knowledge and about our understanding of something as basic as conception.”