The photic sneeze reflex
What this means
Roughly 18–35% of people sneeze when they step from shade into bright sunlight. The trait runs in families and used to be called the "ACHOO syndrome" (Autosomal-dominant Compelling Helio-Ophthalmic Outburst — someone clearly enjoyed naming it). A 23andMe GWAS pinned the strongest signal to rs10427255 on chromosome 2. The most plausible mechanism is mild "cross-talk" between the optic and trigeminal nerves — a bright-light signal accidentally triggers the sneeze pathway.
About one in three people sneeze when they step from shade into bright sunlight. It runs in families and used to be called the "ACHOO syndrome" (Autosomal-dominant Compelling Helio-Ophthalmic Outburst — someone really enjoyed naming it). A large genetic study pinned down one of the strongest DNA changes that predicts it. The most plausible explanation is mild crossed wires between the nerve that senses bright light and the one that triggers sneezes.
Caveats
- Photic sneezing is harmless. Mostly just a fun party trick.
- This is one of several loci linked to the trait — predictive value of any single SNP is modest.
- Driving out of a tunnel into sunlight: aim for two hands on the wheel.
- Mainly studied in European-ancestry participants.