Semi-identical twins 'identified for only the second time'

A chart explaining how different types of twins are conceived

USA Today reports the first set of semi-identical twins were reported in the U.S.in 2007.

Almost all twins are either fraternal (where two eggs and two sperm have created two separate embryos) or identical (where one embryo splits in two, before resuming normal development for each child).

The boy and the girl, now four, from Brisbane, in Australia, are identical on their mother's side.

One of the embryos did not develop but the remaining two went on to grow as twins, each with different genetic material from the father.

"It is likely the mother's egg was fertilised simultaneously by two of the father's sperm before dividing", said Professor Fisk in a press release.

Right along that DNA-sharing spectrum, "semi-identical" twins share anywhere from 50% to 100% of their genomes, researchers say. But they found no other cases of sesquizygotic twins in that sample, nor in worldwide twin studies, Fisk explained. But when the woman came in for a follow-up ultrasound at 14 weeks, it was discovered that she was carrying a boy and a girl-something that is impossible in identical twins.

Franternal twins - those that aren't identical - share half of their DNA, whereas identical twins share all of their DNA.

With semi-identical twins, an egg is fertilized by two sperm, forming a triploid, which then splits in two.

Gabbett said three sets of chromosomes - two from the father and one from the mother - are typically incompatible with life and the embryos don't usually survive.

Some cells contain chromosomes from the first sperm, others hold chromosomes from the second, so the twins will only share some of their paternal DNA - 78 percent in this case.

Doctors in Brisbane discovered the twin's unusual DNA make-up in 2014 while caring for their mother while she was pregnant - the details of which have only just been published.

Australia's Queensland University of Technology has announced that a pair of young Brisbane twins, a boy and girl, have been identified as only the second set of semi-identical twins in the world, and the first to be identified by doctors during pregnancy.

'This is confirming there is this third type of twinning where it's not fraternal and it's not identical.

The girl also developed a blood clot shortly after she was born - blood clots are a common complication for identical twins in general - and the clot cut off the blood supply to her arm.

What you might not know about are sesquizygotic or semi-identical twins, who are neither fraternal or identical, but something in between.

But in this case, the twins did survive and were delivered by cesarean section at 33 weeks of pregnancy, according to the report. In that case, the twins were studied by doctors in infancy after one of them appeared to have ambiguous genitalia.

He and his colleagues examined genetic data from 968 fraternal twins, as well as a number of large global studies, but found no other case of sesquizygotic twins. "While doctors may keep in mind that these types of twins are extremely uncommon, its rarity means there is no case for routine genetic testing".

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