Scientists Find Second Set of Semi-Identical Twins

Desiree Burns
March 1, 2019

Wondering if there were other cases of semi-identical twins that had gone unreported, Gabbett and his team combed through previous medical studies and an worldwide genetic database of 968 fraternal twins and their parents.

It's believed that the twins - now four years old - were formed when one egg was fertilized simultaneously by two sperm, according to Fisk.

The twins only share a portion of their father's DNA and are identical on their mother's side.

But it's not identical from sperm to sperm because each man is a mixture of the genetic material from his parents, and each time a slightly different assortment of that full DNA set gets divided to go into a sperm. They are the first to have been observed in utero (in the womb). A scan at the Royal Brisbane and Women's Hospital in 2014, early in the pregnancy, showed "a single placenta and positioning of amniotic sacs that indicated she was expecting identical twins", Nicholas Fisk, who was head of the mother's care team, explained in a report by Queensland University.

The identity of the twins has not been revealed.

Doctors caught on to the semi-identical twins during the mother's routine ultrasounds. 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.

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Their 28-year-old mother's egg was fertilised naturally by two sperm from the same father - one carrying an X chromosome and one with a Y, researchers revealed. Dr. Michael Gabbett, one of the authors of the case study, said in a press release that "t$3 hree sets of chromosomes are typically incompatible with life and embryos do not 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.

Experts at the Queensland University of Technology said, after fertilisation, the egg split into three embryos with DNA from two sperm. Six weeks into the pregnancy, she underwent an ultrasound that showed her twins were sharing a placenta, an indication that they were identical. Researchers from the University of New South Wales and the Queensland University of Technology combed through almost 1,000 cases of twins to confirm their findings.

Fraternal twins happen when two separate eggs are fertilised by their own sperm. The only other reported case of sesquizygotic twins was reported in the United States in 2007.

"We then started to wonder if over the decades there had been some twins who had been told they were non-identical but in fact could also be three-quarters identical", he said. There's an Aussie set of identical twins who are actually not identical! In that case, the twins were studied by doctors in infancy after one of them appeared to have ambiguous genitalia.

The family declined to be identified, but the twins are doing well and reaching all their development milestones, according to the doctor.

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