Published: Fri, March 01, 2019
Health Care | By Cedric Leonard

Scientists Uncover Second-Ever Case Of 'Semi-Identical' Twins

Scientists Uncover Second-Ever Case Of 'Semi-Identical' Twins

In a report published Thursday in the New England Journal of Medicine, the researchers said they used prenatal genetic testing to identify a set of "semi-identical" twins, USA Today reported. The boy and girl, born in Brisbane, inherited an identical share of their mother's DNA.

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

Next, the three sets of chromosomes would've been divvied up into three separate cells: One cell received chromosomes from the mother and first sperm cell, the other from the mother and second sperm cell and the third from both sperm cells.

"It is likely the mother's egg was fertilized simultaneously by two of the father's sperm before dividing", said Dr. Nicholas Fisk, a professor and maternal-fetal medicine specialist from the University of New South Wales in Sydney.

In 2006, twins in the USA were identified as semi-identical twins when they were examined as infants for another medical condition. So they looked at 968 dizygotic twins to make sure this wasn't the case.

Each sperm cell contains half the father's DNA.

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

In addition, the girl and boy each have male and female sex chromosomes - in other words, each twin has some cells that carry an XX pair (female) and some that carry an XY pair (male).

In the case of the Australian twins, who live in Brisbane and are now 4½ years old, the mother's egg was fertilized with one sperm carrying an X chromosome and one with a Y. Because an ultrasound taken early in the pregnancy showed both fetuses shared the same placenta, doctors assumed the fetuses were identical twins.

It suggests that such twins are incredibly rare, as embryos such as these often do not survive.

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But the 14-week ultrasound showed they were different genders - making it impossible for them to be monozygotic twins.

Fraternal twins are formed when two separate eggs are fertilised by two separate sperm.

A rare set of semi-identical twins have been identified in Australia, only the second set ever reported in the world.

"However, we found no other sesquizgotic twins in these data, nor any case of semi-identical twins in large global twin studies", he said. Researchers from the University of New South Wales and the Queensland University of Technology combed through almost 1,000 cases of twins to confirm their findings.

The only other reported instance of sesquizygotic twins was identified in the United States in 2007.

They were brought to the attention of doctors because one had "ambiguous" genitalia.

The twins were discovered to be semi-identical while their mother was still pregnant.

Twins can be fraternal, identical - and in extremely rare cases - semi-identical.

The girl developed a blood clot in her arm that the doctors think is unrelated to sesquizygosity, Gabbett adds.

The unnamed four-year-olds, however, are said to be "beautiful and healthy".

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