Australian doctors claimed to have identified the second ever one of the semi-identical twins that share 1

00 percent of the mother's DNA, but only part of the DNA of the parent. (Photo: Getty Images / iStockphoto)

A young boy and girl were identified as semi-identical twins, only the second set ever reported in the world, Australian doctors say.

Twins, now four years old, divide 100% of their mother's DNA, but only part of their father's DNA, said a peer-reviewed case report published in the journal New England Journal of Medicine on Thursday.

Identical twins occur when one egg is fertilized with one sperm, then split into two parts, dividing 100% of the DNA of both parents. For brothers twins, each child develops from a single egg, fertilized with its own semen, each of which divides half the DNA of its parents.

"Presumably, the mother's egg was fertilized at the same time with two sperm of the father before dividing," said Nikolai. Fisk, a fetal medicine specialist and vice chancellor of research at the University of New South Wales, in a statement. Fisk assisted in this affair and led a team that was caring for her mother and twins when they were hospitalized in 2014.

The identical or "sesquise" twins were first reported in the United States in 2007.

Australian doctors say that this second set was first found during pregnancy. Fisk said that the mother's ultrasound for six weeks showed that she was expecting identical twins, but the second ultrasound completed at 14 weeks identified them as men and women, which is impossible with identical twins. Michael Gabbeth, a clinical geneticist at the Queensland University of Technology and lead author of the study, said that "sesquiess" embryos usually do not survive, since three sets of chromosomes-one set from the mother and two sets from his father-are created when one egg is fertilized by two sperm, often "incompatible with life".

In this case, the eggs are equally separated by sets of chromosomes, then split. "Some cells contain chromosomes from the first sperm, and other cells contain chromosomes from the second sperm, resulting in the twins sharing only the particle, not 100% of the same parent DNA," Gabbeth said.

Follow Brett Moline on Twitter: @ brettmolina23 .

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This story is: https://www.usatoday.com/story/news/health/2019/02/28/semi- identical-twins-rarity-identified-australia-doctors-claim / 3013228002 /