Step out of a dim cinema into bright afternoon sunshine and watch what happens. About one in four people will immediately sneeze, sometimes two or three times in rapid succession. The rest will simply squint and walk on, mildly puzzled by their companions.

This is the photic sneeze reflex, and it almost certainly runs in your family. If your mother sneezes at sunlight, there's a good chance you do too. It's a small, harmless quirk, but it opens a window onto something fascinating: how a single tweak in your genetic code can create a wiring difference in your brain that follows family lines for generations.

Inside your skull runs the trigeminal nerve, the cable responsible for sensation in your face, including the itchy feeling that triggers a sneeze. Running close beside it is the optic nerve, which carries information from your eyes to your brain. In most people, these two cables stay neatly separated, each minding its own business.

In photic sneezers, something different happens. Researchers believe genetic variants influence how these nerves are bundled and insulated, allowing signals to leak from one to the other. When a sudden flood of light fires the optic nerve, the signal spills sideways into the trigeminal pathway. Your brain, dutifully interpreting the message, concludes your nose is being tickled. So you sneeze.

It's a wonderful example of how genes shape not just visible traits like eye colour, but the microscopic architecture of your nervous system. The instructions written in your DNA decide how neurons grow, where they reach, and how thick their insulation is. Sometimes those instructions produce a tiny shortcut, and you become a person who sneezes at the sun.

Takeaway

Genes don't just build bodies; they wire brains. A single coding variant can rearrange microscopic neural connections in ways that ripple into noticeable, lifelong behaviour.

The photic sneeze reflex follows what geneticists call autosomal dominant inheritance. That sounds technical, but the idea is simple: you only need one copy of the variant gene, from either parent, to inherit the trait. It doesn't get diluted or hidden the way some traits do.

Picture a family tree. If a sneezing father has children with a non-sneezing mother, roughly half the children will inherit his variant and sneeze at sunlight. If both parents sneeze, the proportion climbs higher still. Trace the trait backwards and you'll often find a grandparent, great-aunt, or distant cousin doing exactly the same thing on bright mornings.

Gregor Mendel discovered these patterns in his pea plants in the 1860s, sketching out the rules of dominant and recessive traits without ever knowing what DNA was. The photic sneeze reflex is Mendel's laws still at work, more than a century later, in your own family. The same logic that determined whether his peas were green or yellow is now determining whether you reach for sunglasses or a tissue.

Takeaway

Dominant traits travel through generations in plain sight. You don't have to dig deep into your family tree to find them; just ask who else sneezes at the sun.

Photic sneezing is just one entry in a strange catalogue of inherited reflexes and responses. Some people experience an uncontrollable urge to sneeze after eating a very full meal, a trait charmingly named snatiation. Others get a tingling sensation in their funny bone when they hear a specific sound, or feel a shiver run down their spine when they encounter certain music. Many of these quirks cluster in families.

Then there are the taste oddities. Some people genuinely experience coriander as soapy, thanks to variants in olfactory receptor genes. Others can't smell asparagus in their urine, while their siblings can. Around 70 per cent of people can roll their tongue into a tube; the rest cannot, no matter how hard they try.

Each of these small differences is a clue. Together they remind us that human variation isn't just about the obvious things like height or hair colour. It runs down to the level of how we taste, smell, hear, and even sneeze. Your DNA contains thousands of these tiny dials, each turned slightly differently from the person next to you.

Takeaway

We tend to notice big genetic differences and ignore small ones. But the small ones are everywhere, quietly making each person's sensory world slightly different from everyone else's.

The next time sunlight makes you sneeze, remember: you're watching your genetic code in action, executing an instruction written before you were born and shared with parents and grandparents you may never have thought to ask about.

Genetics is often discussed in terms of disease and risk, but it's also the story of these everyday quirks. They connect us to our ancestors, distinguish us from our neighbours, and remind us that every person carries a slightly different reading of the human script.