Semi-identical twins identified during pregnancy

A boy and a girl in Brisbane have been confirmed as the second set of semi-identical, or sesquizygotic, twins in the world — and the first to be identified by doctors during pregnancy.

Identical twins of course result when cells from a single egg fertilised by a single sperm divide into two, so the twins are the same gender and share identical DNA. Fraternal twins, meanwhile, occur when each twin develops from a separate egg and the egg is fertilised by its own sperm.

But what of sesquizygotic twins?

“It is likely the mother’s egg was fertilised simultaneously by two of the father’s sperm before dividing,” said UNSW’s Professor Nicholas Fisk, who led the foetal medicine team that cared for the mother and twins while based at Royal Brisbane and Women’s Hospital in 2014.

“The mother’s ultrasound at six weeks showed a single placenta and positioning of amniotic sacs that indicated she was expecting identical twins. However, an ultrasound at 14 weeks showed the twins were male and female, which is not possible for identical twins.”

Queensland University of Technology (QUT) clinical geneticist Dr Michael Gabbett said if one egg is fertilised by two sperm it results in three sets of chromosomes, one from the mother and two from the father — and three sets of chromosomes are typically incompatible with life.

“In the case of the Brisbane sesquizygotic twins, the fertilised egg appears to have equally divided up the three sets of chromosomes into groups of cells which then split into two, creating the twins,” Dr Gabbett said.

“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% of the same paternal DNA,” he continued. Indeed, genotyping of amniotic fluid from each sac showed that the twins were maternally identical but chimerically shared 78% of their paternal genome, which makes them genetically in between monozygotic (identical) and dizygotic (fraternal).

Illustration shows two sperm fertilising an egg. Image credit: QUT.

The first known case of sesquizygotic twins was reported in the US in 2007, when one of the twins was identified with ambiguous genitalia in infancy. On investigation of mixed chromosomes, doctors found the boy and girl were identical on their mother’s side but shared around half of their paternal DNA.

Seeking other examples of sesquizygotic twins which had perhaps been wrongly classified or not reported, Prof Fisk and his colleagues analysed worldwide twin databases and examined genetic data from 968 fraternal twins and their parents.

“We found no other sesquizygotic twins in these data, nor any case of semi-identical twins in large global twin studies,” he said, emphasising just how rare sesquizygotic twins are.

“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.”

The case has been reported in The New England Journal of Medicine.

Top image credit: ©stock.adobe.com/au/Robert Hammer

Please follow us and share on Twitter and Facebook. You can also subscribe for FREE to our weekly newsletters and bimonthly magazine.