Off the subject just a hair, but did anyone see that Discovery program called "I Am My Own Twin" about the finding that a very few people apparently started out as fraternal twins in the womb, but one twin 'absorbed' the other and they have two totally separate sets of genes? I kid you not. It was the weirdest thing I have seen in awhile.
Lots of people discover that ---- generally with a tumor that has a eye or tooth in it or something equally disgusting.
Sometimes, though, working parts (e.g., kidneys) are actually genetically different, including one guy whose testes (and thus sperm) were not "his" genetically --- but his absorbed brothers.
Doesn't come close to the embrionic twin on record of a guy who carried his "twin" for 36 years. He looked absolutely pregnant (all photos including the surgery and "afterbirth" were documented) and dropped from 145 to 85 pounds after the surgery. Strangest thing I've ever seen and the details are positively mind boggling. It was on a show that featured the top ten weirdest accidents/surgeries on record.