Chimerism is a genetic trait found in all sorts of animals. It's when you're so close to your fraternal twin you wind up the same organism. Sometimes it does cool stuff like making your eyes different colors or giving you a clearly defined change in hair color at a point. Others get fun little Easter eggs inside our bodies that show up at inconvenient times like people failing the genetic parenting test of someone they gave birth to or extra body parts. It can also cause serious issues if it manifests as mosaicism.