Put that word on your next spelling bee.
The Atlantic did a fascinating piece a few years ago that is new to me. I think it’s stuck behind a paywall, but if you can get it to work, it’s here.
They’d discovered twins who were both fraternal and identical–and different genders.
Definitely only one placenta. Definitely looked like a boy and a girl.
Normally, they explained, if two sperm make it to one egg the result has three sets of chromosomes and is nonviable. What they figured happened was that that happened and survived long enough to split into three: the girl, the boy, and a third part that could not grow.
But.
The girl is 90/10 female.
The boy is 53/47 male.
The split wasn’t evenly boy/girl down the middle and both have parts of each other’s DNA.
Biology not only doesn’t always follow the rules, sometimes it plays Calvinball for the exuberance of joy that it can. To life!