Rare set of semi-identical twins identified in Australia

This illustration shows two sperm fertilizing an egg. (Credit: QUT)

A pair of Australian twins had researchers scratching their heads when it was discovered they were neither identical nor fraternal, but semi-identical siblings.

In a new study published in The New England Journal of Medicine, scientists detail how they identified the set of semi-identical or sesquizygotic twins, only the second known pair in existence, during the mother’s pregnancy in 2014.

At first, the fetal medicine team at the Royal Brisbane and Women’s Hospital believed the 28-year-old woman was going to give birth to identical twins.

“The mother's ultrasound at six weeks showed a single placenta and positioning of amniotic sacs that indicated she was expecting identical twins,” said Dr. Nicholas Fisk, who led the fetal medicine team that cared for the family.

However, when another ultrasound was conducted 14 weeks later, it showed she was going to have a male and a female child – which is not possible for identical twins.

Identical twins occur when cells from a single egg fertilized by a single sperm divide into two, which means they share the exact same DNA and sex.

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.

For the Brisbane twins, researchers were initially puzzled as to how they first appeared to be identical, sharing a single placenta, but then later had different sex.

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.

“The fertilized egg appears to have equally divided up the three sets of chromosomes into groups of cells which then split into two, creating the twins,” said Dr. Michael Gabbett, the first author of the study from Queensland University of Technology in Australia.

“Some of the cells contain the chromosomes from the first sperm while the remaining cells contain chromosomes from the second sperm, resulting in the twins sharing only a proportion rather than 100 per cent of the same paternal DNA.”

Gabbett said three sets of chromosomes – two from the father and one from the mother – are typically incompatible with life and the embryos don’t usually survive.

The Brisbane twins, however, appear to have beaten the odds.

The boy and girl, who are now four years old, are only the second known set of semi-identical twins in the world and the first to be identified by doctors during pregnancy.

“We know this is an exceptional case of semi-identical twins. While doctors may keep this in mind in apparently identical twins, its rarity means there is no case for routine genetic testing,” Fisk said.

The only other reported instance of sesquizygotic twins was identified in the United States in 2007. In that case, the twins were studied by doctors in infancy after one of them appeared to have ambiguous genitalia. The medical team found the American twins’ chromosomes were identical to their mother’s, but not their father's.