Boy and girl, now four, are only the second case of ‘sesquizygotic’ twins recorded

A pair of twins have stunned researchers after it emerged that they are neither identical nor fraternal – but something in between.

The team say the boy and girl, now four years old, are the second case of semi-identical twins ever recorded, and the first to be spotted while the mother was pregnant.

The situation was a surprise to the researchers. An ultrasound of the 28-year old mother at six weeks suggested the twins were identical – with signs including a shared placenta. But it soon became clear all was not as it seemed.

“What happened was the mother came back for her routine ultrasound some months later, and we saw one [twin] to be a boy and one to be a girl,” said Dr Michael Gabbett, first author of the report from Queensland University of Technology in Australia. “At that point we started the genetic studies and worked it out from there.”

Twins are normally either identical or fraternal. In the case of identical, one egg is fertilised by one sperm, but the resulting ball of cells splits in two, giving rise to two offspring with identical genetic material. In the case of fraternal, or non-identical, twins, two eggs are fertilised, each by a different sperm. The resulting siblings arise from the same pregnancy, but are no more genetically similar than siblings from the same parents born at a different time.

Faced with a puzzling scenario, Gabbett and colleagues report in the New England Journal of Medicine that they took samples from the two amniotic sacs, allowing them to investigate the genomes of the twins during the pregnancy.

The results, they say, reveal a very unusual situation: the twins are semi-identical or “sesquizygotic”.

“What we have got is a sequence of unusual events,” said Gabbett.

The situation is believed to arise when two sperm cells fertilise a single egg. In the latest case, one sperm carried an X chromosome among its genetic material, and the other carried a Y chromosome.

After fertilisation the chromosomes from the two sperm cells and the single egg got bundled into three “genetic packages”: one contained chromosomes from both sperm – meaning it contained two sets of genetic material from the father, but none from the mother. The other two packages each contained the same set of chromosomes from the mother, as well as genetic material from one of the two sperm, giving rise to either XX (female) or XY (male) cells.

As the fertilised egg divided and the ball of cells grew, those containing only chromosomes from the two sperm died. However, those containing chromosomes from both the egg and a sperm cell continued to divide.

“Then what happens is that little ball of cells splits into two, and that is why you have twins,” said Gabbett, adding that these offspring have a greater genetic similarity than fraternal twins, but are not identical.

But the situation is not that straightforward: some of both embryos’ cells contained two X chromosomes, while other cells contained an X and a Y chromosome.

Gabbett added since one twin was a boy and the other a girl, the ratio of each of these types of cell differed: one embryo contained a greater number of XY cells, so developed male, while the other had a high proportion of XX cells and developed female.

The only previously reported case of sesquizygotic twins came from the US in 2007, discovered after one of the children was born with ambiguous genitalia. The team say the children in the latest report had no such ambiguities.

However, the case was not without complications. While the children were delivered by caesarean section and appeared healthy, it was found the baby girl had developed a blood clot shortly before birth. Her arm was amputated when she was four weeks old.

Then at the age of three, the girl had her ovaries removed as it was found they had not fully formed, which can increase the risk of a certain type of cancer – a situation that was down to her genetic makeup.

Gabbett said that while there might be other, unreported, cases of sesquizygotic twins, it remains a very unusual phenomenon. Among their investigations, the team looked back at genetic data for nearly 1,000 pairs of what were thought to be fraternal twins, but found no other cases.

“It is incredibly rare so we don’t think it is something people should be routinely tested for or worried about,” said Gabbett. But, he added, it challenges conventional thinking. “We traditionally categorise twins as either identical or non-identical, and this is a third type of twinning characterisation,” he said.