Generally, there are two types of twins in humans – identical and fraternal. But now, researchers have announced that twins born in Queensland, Australia, have been found to belong to an extremely rare in-between type known as "semi-identical" twins. This is only the second known case in the world, and the first time it has been identified during pregnancy using genetic testing.

In the case of fraternal (or dizygotic) twins, two separate eggs are fertilized by two separate sperm, so the resulting embryos share half of their DNA. Identical (or monozygotic) twins form when a single egg, fertilized by a single sperm, splits in two, creating embryos that share 100 percent of their DNA and are always the same sex.

In extremely rare cases there is a third intermediate form, known as semi-identical (or sesquizygotic) twins. This occurs when one egg is fertilized by two sperm at the same time, splitting into two the same way as identical twins but each carrying different genetic information from the father.

As a result, semi-identical twins share three quarters of their DNA – 100 percent from the mother's side, since they both hail from the same egg, but a different amount matching on the father's side, since they came from two different sperm.

While fraternal and identical twins account for roughly a few dozen births per thousand, semi-identical twins are incredibly rare. The first known case occurred in the US in 2007, making the newly-announced Australian babies only the second instance.

Last time, the fetuses were assumed to be identical twins during pregnancy, and were only identified as semi-identical after birth, when one of the babies was found to be intersex. In this new case, specialists were able to identify the twins as sesquizygotic while in the womb.

"The mother's ultrasound at six weeks showed a single placenta and positioning of amniotic sacs that indicated she was expecting identical twins," says Professor Nicholas Fisk, lead researcher on the study. "However, an ultrasound at 14 weeks showed the twins were male and female, which is not possible for identical twins."

To confirm the discovery, the scientists then conducted genetic testing on the amniotic sacs of each fetus. They found that while they shared 100 percent of their mother's DNA, they only shared 78 percent of their father's, making them semi-identical. In total, their genomes were 89 percent identical.

Normally, if two sperm fertilize one egg, three sets of chromosomes are introduced – one from the egg and one from each sperm. But life usually can't develop that way, so these embryos wouldn't survive in the vast majority of cases – hence the extreme rarity of this type of twin.

"In the case of the Queensland 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," says Michael Gabbett, another author of the study.

The team then investigated just how rare semi-identical twins may be. After analyzing databases of twin births worldwide they found no other cases besides the two known instances.

"We at first questioned whether there were perhaps other cases which had been wrongly classified or not reported, so examined genetic data from 968 fraternal twins and their parents," says Fisk. "However, we found no other sesquizygotic twins in these data, nor any case of semi-identical twins in large global twin studies."

The twins themselves are now four years old and reportedly happy, healthy and developing normally.

The research was published in The New England Journal of Medicine. The team describes the work in the video below.

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