I've recently been getting back into reading the series after stepping away for a number of years. Even though I'm supposed to be on a writing vacation, this one teeny-tiny idea has been at the back of my mind for the last two days, threatening to blossom into a short story. It's kinda maddening, but also inspiring; that's just the writer's curse for you though.

Genetic chimaerism can be the result of a mutation of genetic material in a cell causing an organism bears more than one distinct genetic line in certain subsets of cells (known as genetic mosaicism), or through the absorption/fusion of two or more fertilised zygotes in utero (for example, 'vanishing twin syndrome'). Many people who experience it may never find out they even have it unless they are tested. In the past, there was a woman at risk of losing her children when tests 'showed' they weren't hers, until a lawyer who had somehow heard of chimaerism suggested she be checked for that.

Although that woman ultimately kept her children when it was proven she had chimaerism, it made me wonder a few things, especially in regards to how Equine Transformation Syndrome would behave. A decent portion of cells must be of the 'twin subset', as heterochromia iridium—and even rarer cases of hair being different colours in patches—can be caused by mosaicism and chimaerism; although I wouldn't expect anyone to have percentage estimates, how do you reckon ETS would react to someone who had both male and female genes as a result of absorbing their fraternal twin?

Would it simply follow the trend of 'cure a person of most physical disorders, disease, and chronic conditions'? Or might it be a possible consideration Sunset Shimmer did not make, resulting in the spell having unpredictable results? If so, I could see the potentiality for turning one human into two separate ponies. Then of course, would that become a single mind spread across two bodies (because magic), or two separate copies of the same mind? In that case, would it end up turning an adult into two foals or two adults?

I'd like to hear your thoughts on the matter.

Well it would probably depend on how big the original person was, assuming it did split into to people. Conservation of mass is somewhat loose I believe in this universe, but considering both of these people would only have like half a mind, they would be…foals? A mind spread over two bodies seems a bit unlikely to me, but ETS acts in strange ways I suppose, two copies seems more likely that either of the two above though. It would be a nice story idea to flesh out farther.

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7874664
I agree conservation of mass seems like the biggest issue here. Mass can be taken from other sources since rehumanization requires it to some extent, but you are speaking on a whole different scale. Then there would be how the two would end up separated. Sunset Shimmer would have been unlikely to have considered the condition, but I'm unsure how it would play out.

It is an interesting concept. If you can think of a way of making it work, go ahead and write ot out.

7874679
I think by Day 6 onward, it'd probably get to a point where the body-horror aspect of the transformation might be a bit more on the extreme side as well. Especially if ETS is constructing bodies by dividing one into two. Might be even worse psychologically if one half were a night pony, since then sleep deprivation could become a problem if their night pony and non-night pony parts conflict.

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The new would either mirror the other or would be likely an earth pony. It's a blank slate as far as ETS is concerned. If it didn't mirror the original the determinant for ETS sorting would just default to earth pony.