Never say never in medicine and biology.
"We think that the egg probably carried an X and one sperm an X, and one sperm a Y. It's possible that it is far more complicated than that. But that's the simplified explanation," says Souter. The twins have different proportions of male cells (XY) and female cells (XX), and are chimeric, meaning they have tissue with a diverse genetic make-up. Twin A, identified as a hermaphrodite, has 5% XY and 95% XX after genetic testing of the skin. Comparatively, Twin B, the male, has 50% XX and 50% XY.
Weird science but cool. (like Robert Downey Jr.)
March 30th, 2007 12:33am
Does this mean they're going to turn into houseflies when they get older?
March 30th, 2007 9:51am
I suspect that chimeras  are far more common than previously thought. About 1 in 20 to 1 in 25 newborns have ambiguous genitalia at birth , and are whisked away to have some quick surgery before the parents cotton on.
When researchers were trying to determine why in-vitro (test-tube) embryos had a 20% survival rate (80% died before live birth), they wondered what the "old fashioned" survival rate was. The in-vivo (old fashioned) embryos had a 40% survival rate . My suspicion is that twins are far far more common than thought, and twins are more likely the norm than single conceptions. On some occasions, the embryos merge together forming chimeras. Only widespread systematic DNA testing will be able to determine just how common chimeras are. At the moment, very few people get DNA samples taken. Other than the military and prison population (where 100% get DNA samples taken), in the general population it is in the 5% ballpark.
1 - a chimera is when 2 embryos fuse together. Sometimes it is detectable as blood tests showing more than one result. Sometimes it is detectable as DNA testing from different parts of the body showing different results. There is at least one criminal case where DNA samples from cheek swabs showed negative, and semen samples showed positive in a rape case.
2 - where you can't immediately say "its a boy" or "its a girl" and instead have to look closer.
3 - if "life" begins at conception, as anti-abortion proponents claim, then why does 60% of the human race die before birth?
March 30th, 2007 9:51am
Chimerism is what that Tour de France cyclist winner is claiming. Says when they found different blood in him during the race it wasn't because of doping but because they hit a different, um, vein than last time.
Biology is cool. Categories are not as clear cut, as quantum, as we humans come to expect. In this case the categories of man/woman and those of you/me are mixed up at the same time.
March 30th, 2007 11:07am