Tetragametic chimerism

I have a riddle for you: "The patient, who we will call Jane, needed a kidney transplant, and so her family underwent blood tests to see if any of them would make a suitable donor. When the results
came back, Jane was hoping for good news. Instead she received a hammer blow. The letter told her outright that two of her three sons could not be hers."
Well if you've paid attention to the title of this post you probably already know the answer. Jane is a chimera. She is a mix of cells from two twin sisters, conceived from two pair of gametes, that somehow got mingled up into one person. There are 30 or so reported cases of true tetragametic chimerism in the literature, often because they lead to hermaphroditism when the chimera is of a male and a female twin. However many more lurk around us without knowing. It may be just a few cells in the blood, or a patch in one organ, and you would probably never know...except from when you get blood type or genotype discrepancies. Such a case actually popped up recently when Tyler Hamilton was charged with blood doping after competing in a cycling race. He had two blood types in his veins, but it wasn't because of blood transfusion, he is a chimera. And the prevalence is mind boggling: 20-30% of the pregnancies that start out as twins end up as a single baby, and nearly 70% of all people may be chimeras. Although not everyone agrees with these statistics, it may explain, weird auto-immune disorders. And perhaps some transsexual people really are a girl trapped in a man's body, literally. here.