The Sun Sneeze Gene

The photic sneeze reflex is a condition where going from a dark area into bright light causes sneezing, also known as autosomal dominant compelling helio ophthalmic outbursts syndrome.

The presenter demonstrates the reflex by looking at the sun and sneezing as a direct example.

People have known about this effect for at least a few thousand years, with Aristotle asking about it in his book of problems.

Why does this biological response happen when exposed to bright light?

Heat from the sun causes sweating inside the nose, leading to sneezing to remove the moisture.

Sir Francis Bacon demonstrated Aristotle's theory was wrong by closing his eyes toward the sun and not sneezing, proving the eyes are involved.

Bacon proposed that eye watering causes moisture to reach the nose, creating a tickle that triggers sneezing.

Eye watering is a much slower process than the photic sneeze reflex, so it cannot be the actual cause.

The presenter initially thought having the photic sneeze reflex must convey an evolutionary advantage.

• Sneezing transmits pathogens like bacteria (tuberculosis, strep throat) and viruses (measles, mumps, rubella, influenza) • In dark caves, mucus remains viable for hours or days, spreading disease to cave dwellers • Sneezing in bright sunlight causes mucus to dry quickly or be killed by UV rays

Sneezing only when emerging from darkness into bright sunlight greatly decreases disease transmission risk.

Since only 18 to 35 percent of people have the trait, it likely doesn't provide a significant evolutionary advantage and may be a random mutation.

• A father who sneezed exactly twice in bright sunlight had a four-week-old daughter who sneezed exactly twice when moved from dark to sunlight • By the 1980s, the trait was identified as autosomal dominant, requiring only one copy from one parent

The gene responsible for the photic sneeze reflex is on the second chromosome, with a single DNA letter change where sun sneezers have a C and non-sun sneezers have a T.

A 2010 study of 10,000 people who reported their sun sneeze status online had their DNA analyzed, revealing this specific letter change common to all sun sneezers.

The best current theory involves the trigeminal nerve, which has three branches receiving stimuli from the eye and nose, with cross-activation between branches triggering the sneeze.

Photic sneezes are generally harmless except in dangerous situations like flying a fighter jet or performing delicate surgery.

Studying large populations and comparing genotypes allows researchers to learn about heritable traits and diseases.

• 23andMe identified the gene polymorphism responsible for the photic sneeze reflex • The company's name comes from humans having 23 pairs of chromosomes • It provides personal DNA insight by analyzing saliva samples

Results reveal ancestry, photic sneeze reflex status, health information, physical traits, and Neanderthal genetic percentage.