Ahh, I see. They're 78% identical in paternal DNA because the same set of paternal DNA got into both embryos, not because two sperm were exceptionally similar.

This is underplayed by the terminology "semi-identical twins, with the same maternal DNA but different paternal DNA". They are significantly more identical than that. I bet twins actually matching that description wouldn't have the developmental problems these have apparently already manifested.

I wonder if the boy might have any symptoms of Klinefelter syndrome? (On the assumption that the two sperm were an X and a Y.)

> They mentioned a 3rd embryo not being viable as it only contained sperm DNA.

The CBC journalist misunderstood what the research article was saying. They did not detect Y1-Y2 cells, but cite research on bovines zygotes that found that in addition to the X-Y1 and X-Y2 cell lines, a Y1-Y2 cell line will also form, but this cell line will "undergo growth arrest before somitogenesis", accounting for why they did not find such cells in these twins. So the zygote can be presumed to have had cell lines with three different detectable gene combinations, but the paternal-paternal line cells are entirely or at least mostly terminated very early in development. Figure 3 in the original paper is a really nice illustration of how this proceeded. There was never a separate paternal-paternal embryo, contrary to the journalist's misunderstanding, but there likely was a separate paternal-paternal cell line. It's conceivable there could still be trace Y1-Y2 cells in both twins (just as both have X-Y1 and X-Y2 cells), but they are below limits that were detectable in the study and most likely they all were arrested from further division and died off.