Semi-identical twins identified during pregnancy using genetic testing

Henrietta Brewer
March 1, 2019

In the case of fraternal or non-identical twins, two eggs are fertilized by different sperm during the same pregnancy and the children have separate genetic data, like siblings from the same parents born at separate times.

The report also included a video with some helpful animations to illustrate how semi-identical twins occur. Six weeks into the pregnancy, she underwent an ultrasound that showed her twins were sharing a placenta, an indication that they were identical.

After studying the two amniotic sacs, the medical team was able to determine the twins were sesquizygotic, which occurs when one egg is fertilized by two sperm at the same time.

Doctors have discovered a set of twins who were born from one egg and two different sperm.

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.

"We found no other sesquizygotic twins in these data, nor any case of semi-identical twins in large global twin studies", he said.


The only other known case of semi-identical twins was two born in the United States in 2007. They are the first worldwide to be diagnosed by genetic testing while still in the womb.

The only other reported case of sesquizygotic twins occurred in the U.S. in 2007, when the boy and girl were infants.

The 4-year-old boy and girl from Brisbane share all of their mother's DNA but only a part of their father's DNA, making them identical on their mother's side but fraternal on their father's, a statement detailing the discovery said.

To get a better understanding of how rare sesquizygosis twins are, the researchers went on to examine the genes of almost 1,000 other twins and didn't find another case of semi-identical twins, nor did they find evidence of the condition in a previous study from another research team that examined DNA from more than 20,000 twins.

"I was a bit cheeky and accused Professor Fisk of having inadequate ultrasound skills, but turns out they were perfectly good", Gabbett told the Australian Broadcasting Corporation.

Scientists understand biologically how the DNA of our mother and father mix to create an embryo (and subsequently a baby).


Right along that DNA-sharing spectrum, "semi-identical" twins share anywhere from 50% to 100% of their genomes, researchers say. The only other reported case was uncovered in 2007.

"However, an ultrasound at 14 weeks showed the twins were male and female, which is not possible for identical twins". Doctors identified them when one baby had ambiguous sex organs.

The case is being reported on in The New England Journal of Medicine.

"While doctors may keep this in mind in apparently identical twins, its rarity means there is no case for routine genetic testing".

As a result, the fertilised egg contains three sets of DNA chromosomes - two from the father and one from the mother.

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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