Semi-identical twins have been discovered for only the second time

Grant Boone
March 1, 2019

The first case was reported in the United States but the latest occurrence, reported on in the The New England Journal of Medicine, is the first to have been identified during pregnant. "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". Researchers say they're unusual because they share 100 percent of their mother's DNA, but only some of their father's.

Non-identical twins, or dizygotic twins, can share completely different genetic features.

Michael Gabbett, a geneticist at Queensland University of Technology, and one of his co-authors, Nicholas Fisk, an obstetrician and deputy vice-chancellor of research at the University of New South Wales, oversaw the twins' fetal care back in 2014 at the Royal Brisbane and Women's Hospital.

What is a semi-identical twin?

The identity of the twins has yet to be revealed. On investigation of mixed chromosomes, doctors found the boy and girl were identical on their mother's side but shared around half of their paternal DNA.

But what happens when two sperm make it to the same egg? "If it does get in there, you should have three, rather than two sets of chromosomes, which means the embryo is usually not viable".

The twin boy and girl were found to have 100% of their mother's DNA in common, but were only 78% identical in the paternal DNA they carry.

"The mother's ultrasound at six weeks showed a single placenta and positioning of amniotic sacs that indicated she was expecting identical twins", he said in a statement.

Of course, researchers were curious about their uncommon DNA and began doing research into other occurrences of semi-identical twins around the world and were amazed to discover these twins were only the second recorded case in the world, and the first to be identified during pregnancy.

Identical twins occur when one egg is fertilised by an X or Y sperm and divides, forming a pair of twins sharing the same gender and other similar traits - including hair colour, eye colour and facial features.

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The first documented case of half-identical twins was in the USA in 2007.

Prof Fisk said the twins, who are healthy and developing well, were a rarity.

Sesquizygotic represents a third type of "twinning" between identical and fraternal.

This comes just over a month after twins born through an advanced IVF treatment had two different biological fathers.

The only other reported instance of so-called sesquizygotic twins was identified in 2007, brought to the attention of doctors because one had ambiguous genitalia.

It's thought that if two sperm simultaneously fertilize the same egg, the resulting cell with three sets of chromosomes will not be viable.

To see if the phenomenon might be more common than doctors believed, the Gabbett team examined an worldwide database of 968 fraternal twins and their parents.

Professor Fisk added: "We know this is an exceptional case of semi-identical twins".

The authors also performed genetic tests of 968 sets of other twins presumed to be fraternal to see if any were really semi-identical.

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