A young girl underwent a DNA test to establish her paternity. The results that came back were so confusing that the science director of the genetic institute in Colombia, Juan Yunis, assumed the sample was contaminated and ordered the test done again. But the results were the same. Some parts of her genome excluded her mother as a parent, while others excluded her assumed father. Further tests found that the child's mother had XY chromosomes in her blood, which indicate a genetic male.
Further research showed that the girl's assumed father was, in fact, her biological father, but only when the mother's DNA was excluded. That drew Yunis' attention to more thoroughly test the mother. The mother had XY chromosomes in her blood and saliva, but her hair and cheek cells had XX chromosomes. Parts of the daughter's genome matched each kind of her mother's mismatched DNA. The daughter had inherited some DNA from her mother which originally belonged to her mother's fraternal twin brother, who was never born. That makes the mother a chimera, the result of an embryo that had absorbed and incorporated cells from a twin who had vanished before anyone knew he had existed. Read the convoluted way this all came about, and how it was found, at Grid. -via Digg
(Image credit: DimitrisSideridis)