In human biology, a chimera is an organism with at least two genetically distinct types of cells -- or, in other words, someone meant to be a twin. But while in the mother's womb, two fertilized eggs fuse, becoming one fetus that carries two distinct genetic codes -- two separate strands of DNA.

The twin is invisible, but for chimeras the twin lives microscopically inside the body as DNA.

*speechless*

I've heard about chimeras before and it's a very rare condition. A few seasons back the original C.S.I. show did an episode about it.

Fascinating thanks