Imagine trying to photocopy a pile of papers, only for one of the copied sheets to magically jump back into the queue. It gets duplicated again. When the photocopier is finished, you’re left with two sets of papers and three copies of the mysteriously mobile sheet.
The same thing happens in the cells of a fly. Every time a cell divides, it duplicates its entire genome so the two daughter cells each have a copy. But some genes aren’t content to be duplicated just once. A selfish gene called a P-element has the ability to jump around its native genome. Like the paper jumping back into the photocopier queue, the P-element lands in parts of the fly genome that haven’t been copied yet. This ability allows it to spread throughout a genome, and even around the world.