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"In addition, the girl and boy each have male and female sex chromosomes — in other words, each twin has some cells that carry an XX pair (female) and some that

"In addition, the girl and boy each have male and female sex chromosomes — in other words, each twin has some cells that carry an XX pair (female) and some that carry an XY pair (male). Having both male and female pairs of sex chromosomes throughout various cells in the body is linked to certain developmental problems in the reproductive organs as well as cancer, Gabbett said. When the doctors examined the girl twin's ovaries, they found some changes linked to cancer, and so "the difficult decision was made to remove them so she didn't develop cancer," he said. The girl also developed a blood clot shortly after she was born — blood clots are a common complication for identical twins in general — and the clot cut off the blood supply to her arm. As a result, doctors also had to amputate her arm." "When, in a separate unlikely event, the blastocyst divided into two – creating twins – more of the cells with genetic material from one sperm (which had an X chromosome) ended up in one embryo, while the other had a greater proportion of cells formed by the other, Y chromosome bearing sperm. This is why one fetus developed female characteristics and the other male, although both twins have cells from both sperm in their bodies, making them chimaeras"

thaumasiotes 8y ago
> When, in a separate unlikely event, the blastocyst divided into two – creating twins – more of the cells with genetic material from one sperm (which had an X chromosome) ended up in one embryo, while the other had a greater proportion of cells formed by the other, Y chromosome bearing sperm. This is why one fetus developed female characteristics and the other male Rather than the male embryo having more Y-chromosome bearing cells overall, I think it's more likely that the male embryo developed that way because the cells that became the gonads -- as opposed to the majority of all cells -- had Ys. Sexual development isn't done by majority vote.
SamBam 8y ago
Oh, interesting. That was totally missing from the original article. I had assume that the twinning was because the egg had been fertilized by two sperm cells. Rather, it seems that it's quite possible for any single-birth pregnancy to involve two sperm cells, and would result in a chimera. The fact that this one split into twins was a separate event.
- thaumasiotes 8y ago
  The really simple model for an egg being fertilized by two sperm is that you'd get one fertilized zygote with three sets of chromosomes, fully triploid. Such a zygote would miscarry pretty quickly. To get a less nonviable chimera, you need the egg to divide exactly once, such that one half gets one of the sperm and the other half gets the other one. If there is a mechanism that tends to ensure this happens, then it feels like that mechanism might also be related to the twinning (tripling, apparently) of the zygote under discussion here.

thaumasiotes 8y ago

> When, in a separate unlikely event, the blastocyst divided into two – creating twins – more of the cells with genetic material from one sperm (which had an X chromosome) ended up in one embryo, while the other had a greater proportion of cells formed by the other, Y chromosome bearing sperm. This is why one fetus developed female characteristics and the other male Rather than the male embryo having more Y-chromosome bearing cells overall, I think it's more likely that the male embryo developed that way because the cells that became the gonads -- as opposed to the majority of all cells -- had Ys. Sexual development isn't done by majority vote.

SamBam 8y ago

Oh, interesting. That was totally missing from the original article. I had assume that the twinning was because the egg had been fertilized by two sperm cells. Rather, it seems that it's quite possible for any single-birth pregnancy to involve two sperm cells, and would result in a chimera. The fact that this one split into twins was a separate event.

thaumasiotes 8y ago
The really simple model for an egg being fertilized by two sperm is that you'd get one fertilized zygote with three sets of chromosomes, fully triploid. Such a zygote would miscarry pretty quickly. To get a less nonviable chimera, you need the egg to divide exactly once, such that one half gets one of the sperm and the other half gets the other one. If there is a mechanism that tends to ensure this happens, then it feels like that mechanism might also be related to the twinning (tripling, apparently) of the zygote under discussion here.