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Medical rarity: medical team reveals new type of twins


Twins who are neither monozygotic nor dizygotic

For the first time in the history of medicine, a team of doctors discovered an extremely rare form of human twins during pregnancy. These are neither monozygotic nor dizygotic, but semi-identical. Only another pair of twins of this type is known worldwide. The medical team that looked after the mother and twins explain what makes these twins so special.

Semi-identical twins are created when two sperm fertilize an egg at the same time. Usually this leads to the egg cell not developing any further. But in this extremely rare case, the semi-identical twins survived and are now four years old. This medical sensation was recently described in the "New England Journal of Medicine".

What makes the case so special

The case from Brisbane, Australia, attracted worldwide attention. For the first time, doctors were able to see how two embryos develop from three genes. In a normal birth, the embryo divides the genetic material from mother and father into equal parts. In identical twins, after fertilization, the embryo is divided into two completely identical parts. When dizygotic twins were born, the mother had two eggs that were fertilized by different sperm.

Twins from three genes

Nature can deal with these peculiarities. However, if two sperm fertilize an egg at the same time, there is a special case. The father's genome is now twice present and the mother's genome once. "Three sets of chromosomes are usually incompatible with life and embryos don't usually survive," explains Dr. Michael Gabbett in a message. As a clinical geneticist and coordinator for diagnostic genomics at the Institute of Health and Biomedical Innovation, he examined the exceptional case.

Astonished medical team

"The mother's ultrasound showed a single placenta after six weeks," said Gabbett. At first, the doctors suspected that they were identical twins. At week 14 of pregnancy, however, it turned out that one of the twins is female and the other is male. However, this is not possible with identical twins.

Same genetic material of the mother - different from the father

Therefore, the twins' DNA was examined in the womb. The doctors found that the embryos shared 100 percent of the mother's genetic makeup. However, the siblings received different genetic information from the father's side. Thus, the semi-identical twins form a third and extremely rare type of twin.

First discovered in the USA

This type of twins was first discovered in the United States in 2007. They grew up in infancy because one of the twins had no clear gender. After a genetic test, it also emerged that the twins' DNA was identical on the mother's side, but they only share around half of the father's DNA.

Only two semi-identical pairs of twins known worldwide

Since this case became known twice, the medical team wondered if there were more such semi-identical twins. They examined a database of genetic data from 968 twins and their parents, but were unable to find another case. This makes the semi-identical twins and those from the USA the only known pairs of twins of this type in the world. (Vb)

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