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Scientists stunned by discovery of 'semi-identical' twins | Science | The Guardian
2019-02-27, by Nicola Davis,

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

"“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."

"(...) 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. (...)"
birth  gender  twin  science  genetics 
23 days ago by eric.brechemier

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