First semi-identical twins identified during pregnancy

Brisbane's young twins, a boy and a girl, have been identified as the second group of semi-identical, or sesquizygotic, twins in the world – and the first to be identified by doctors during pregnancy.

• The boy and the girl, now four years old, are identical (monozygotic) on their mother's side, sharing 100% of their mother's DNA, but they are like brothers and sisters on their father's side, only sharing a part of their father's DNA.
• The case, the first in the world to identify semi-identical twins on genetic tests in the uterus, has been reported today. The New England Medical Journal (NEJM) by UNSW Fetal Medicine Specialist and Deputy Vice Chancellor (Research), Professor Nicholas Fisk, and Dr. Michael Gabbett, Clinical Geneticist and Clinical Geneticist of Queensland University of Technology (QUT).
• Sesquizygotic represents a third type of "twinning" between the identical and the fraternal (dizygotic).

"It is likely that the mother's egg was fertilized simultaneously by two of the father's spermatozoa before division," said Professor Fisk, who headed the fetal medicine team that was busy mother and twins while she was based at Royal Brisbane and Women's Hospital in 2014. Professor Fisk, former president of the International Society of Fetal Medicine and Surgery, worked alongside the Dr. Gabbett.

"The mother's ultrasound at six weeks showed a single placenta and amniotic sac positioning indicating that she was waiting for identical twins." However, an ultrasound at 14 weeks showed that the twins were men and women, which is not possible for identical twins. "

Identical twins result from the split in two of the cells of a single egg fertilized by a single sperm. Thus identical twins belong to the same sex and share identical DNA. Fraternal twins occur when each twin develops from a separate egg and the egg is fertilized by its own sperm.

Dr. Gabbett explained that if an egg is fertilized by two spermatozoa, three sets of chromosomes result, one from the mother and two from the father.

"Three sets of chromosomes are generally incompatible with life and embryos do not usually survive," he said.

In the case of Brisbane's sesquizygotic twins, the fertilized egg also appears to have divided the three sets of chromosomes into groups of cells that then split into two, creating the twins.

"Some of the cells contain the chromosomes of the first sperm, while the remaining cells contain chromosomes of the second sperm, so the twins share only a proportion rather than 100% of the same paternal DNA."

Sesquizygotic twins were first reported in the United States in 2007. These twins have been reported to physicians since childhood, when one of them had been identified with ambiguous genitalia. In search of mixed chromosomes, the doctors found that the boy and the girl were identical on their mother's side but shared about half of their paternal DNA.

Professor Fisk said that an analysis of twin databases around the world has shown just how rare the sesquizygotic twins are.

"We first wondered if there were possibly other misclassified or undeclared cases, so we looked at the genetic data of 968 fraternal twins and their parents," he said. he declared.

"However, we found no other sesquizygous twin in these data, nor any cases of semi-identical twins in the major global twin studies.

"We know that it's an exceptional case of semi-identical twins.While doctors keep this in mind in seemingly identical twins, its rarity means that it's not the same." there is no reason to carry out routine genetic tests. "

The document, Molecular management of heterogony resulting from sesquizygotic twinning, is published in The New England Medical Journal February 28th.

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