or are they always identical?
Conjoined twins are always identical. They come from a single egg that splits. In this case, the egg doesn’t split all the way.
Fraternal twins come from two separate eggs and are never joined to begin with.
When I read this, I wondered if fraternal twins ever implant so close as fertilized eggs that their cells grow up against each other and commingle as they divide, thus creating conjoined fraternal twins. But then I thought “If conjoined twins’ DNA wasn’t identical, wouldn’t their immune systems start attacking each other and eventually kill one or both of them?”
It can be supposed that there could be chimeric conjoined twins.
Right, they are never joined ***to begin with, ***but there have been cases of the joining of fraternal embryos. I’m looking for a cite.
This pretty much covers it, but it is rare among humans (they cite 40 reported cases) and they don’t mention any where there were two distinct beings partially joined together. Just a single person with the genetics of two people. I can’t find anything about two partially joined embryos…where the second embryo wasn’t completely absorbed.