Don: Hey Yael, are you a twin?

 

Yael: Nope.

 

D: Are you sure?

 

Y: Unless there's something my parents haven't told me, yes, I'm sure. I'd know if I had a twin brother or sister.

 

D: Perhaps, but even if you don't have a twin, you could still be a twin.

 

Y: I don't get it.

 

D: You could be a chimera.

 

Y: Oh, right. Actually, I still don't get it. A chimera is a mythological beast with the head of a lion, the body of a goat, and the tail of a dragon, right?

 

D: In mythology, yes, but in biology a chimera is an organism made up of two distinct genetic lines.

 

Y: But how is that possible?

 

D: It's what happens when twin embryos fuse in the womb. The fusion results in what looks like a single embryo, but the genetic material from each twin remains separate.

 

Y: So what does that mean? Does a chimera have one arm from one twin and the other arm from the other twin?

 

D: Something like that. It's not like the arms would look different, exactly, or that the  body's organs wouldn't be compatible or not work properly. But they do contain different and distinct sets of chromosomes. So if you're a chimera, your liver could  be composed of cells with one set of chromosomes while your heart, say, consists of  cells with an entirely different set. Cool, huh?

 

Y: What happens when the embryos that fuse are of different sexes?

 

D: Then the chimera could be a hermaphrodite.

 

Y: Well, I know I'm not a hermaphrodite.

 

D: But you could be your own twin.