We’re slowly beginning to unpick the complex interplay of genes, environment and experience that make you who you are – and like no one else who ever existed
9 December 2020
How likely are you?
CHILDREN are generally fascinated by tales of how they came to be. Even young ones can often grasp the mind-boggling implication if the events of the story leading up to their existence had been any different: they wouldn’t be there to hear it.
Your you-ness is a precarious thing. Rerun the experiment of you with a different sperm and egg from the same people, and “you” would be as different from your current self, genetically, as siblings are from one another. If the egg were the same, but through some random fluctuation a different sperm won the race, you would also be distinctly different. For a start, depending on whether the sperm bore an X or a Y chromosome, you could have ended up another sex. “That’s a pretty big difference, right there,” says David Linden, a neuroscientist at Johns Hopkins University in Maryland and author of Unique: The new science of human individuality.
The potential for being a different you didn’t stop once destiny set your founding sperm and egg on their collision course, either. A lot of what makes you what you are is down to how your brain is connected. But your DNA doesn’t encode a precise wiring diagram: it is more like a rather hand-wavy recipe or set of instructions. Even genetically identical twins don’t end up with the same neuronal network. “A pool of cells in the developing brain might receive instructions that say: ‘About half of you move across the midline of the brain’, ” says Linden. “In one twin, 40 per …