Boy and girl, now four, are only the second case of ‘sesquizygotic’ twins recordedA pair of twins have stunned researchers after it emerged that they are neither identical nor fraternal – but something in between. An ultrasound of the 28-year old mother at six weeks suggested the twins were identical – with signs including a shared placenta. The results, they say, reveal a very unusual situation: the twins are semi-identical or “sesquizygotic”. 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. “We traditionally categorise twins as either identical or non-identical, and this is a third type of twinning characterisation,” he said.

Source: The Guardian February 27, 2019 21:56 UTC