Here's how it happened: Two sperm cells fertilized one egg—an event assumed to be very rare—then split into two embryos.Link (via Fortean Times). Aha! So two sperm can fertilize an egg at once. Yet another point on which my middle school sex ed teacher was wrong.
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The semi-identical twins only came to the attention of Souter and her colleagues because one had ambiguous genitalia. The child was born a "true hermaphrodite" with both ovarian and testicular tissue. The other twin is a male, anatomically.
She also told the class that it was impossible for a man to urinate while he has an erection. At which I wondered, Oh yeah? Then what am I doing in my pants right now?
Indexed by tags science, nature, medicine, genetics, twins, semi-identical, fraternal, identical, sex ed, too much information.
Image credits: "New York Minute," courtesy Ahafilm, borrowed for news-reporting and comment purposes.