A rare group of semi-identical twins – only the second group ever reported in the world – has been identified in Australia.
The boy and the girl of Brisbane, 4 years old, share all the DNA of their mother, but only part of that of their father, making them identical on the maternal but fraternal side of the paternal parent, indicates a statement relating this discovery .
It was also the first time that semi-identical twins were identified during pregnancy, according to a case report recently published in the New England Journal of Medicine.
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Semi-identical twins, also known as "sesquizygotic twins", were first discovered in the United States in 2007.
"It is likely that the mother's egg was fertilized simultaneously by two of the father's spermatozoa before division," said Nicholas Fisk, a professor at the University of New South Wales in Australia, in a statement. online. Fisk is a fetal medicine specialist who led the fetal medicine team that looked after mothers and twins and worked with the clinical geneticist and lead researcher, Dr. Michael Gabbett, on the case report.
When an egg is fertilized by two sperm, it creates three sets of chromosomes, "one of the mother and two of the father," Dr. Gabbett explained.
"Three sets of chromosomes are generally incompatible with life and embryos do not usually survive," he added. "In the case of sesquizygous twins from Brisbane, the fertilized egg seems to have also divided the three sets of chromosomes into groups of cells that then split into two, thus creating the twins."
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The doctors caught the semi-identical twins during the mother's routine ultrasound examinations. Her six-week ultrasounds indicated that she was expecting identical twins, but the 14-week analysis "showed that the twins were men and women, which is not possible for twins." identical, "said Fisk.
The 28-year-old woman was a mother for the first time and had conceived naturally, reported the BBC.
To ensure that there were no other semi-identical twins that had not been identified before, the doctors examined the genetic data of 968 fraternal twins and their parents, but found no another example of the phenomenon.
"We know that there is an exceptional case of semi-identical twins," Fisk said. "While doctors can keep this in mind in seemingly identical twins, its rarity means that there is no case for a routine genetic test."