Or, the autosomal dominant compelling helio ophthalmic outbursts syndrome.

Which basically means if I go from a dark area into somewhere that's brightly lit - You know, like, looking at the sun - I will sneeze.

Wait for it...

There you go, I'm a sun sneezer.

But why does this happen?

People have known about this effect for at least a few thousand years

Aristotle was probably a sun sneezer because in his book of problems

he asked: Why does the sun provoke sneezing?

And his answer was it was the heat from the sun which causes

sweating inside your nose, and so you sneeze to get rid of the moisture.

but a couple millennia later

Sir Francis Bacon demonstrated that this could not be the case because when he closed his eyes and turned towards the sun

well he did not experience that photic sneeze reflex so his explanation was that it was the eyes

watering and then when some of that moisture got down into the nose that caused the tickle which makes you sneeze

but even this theory has its problems because eye watering is a much slower process than the photic sneeze reflex

So it can't be the cause. Before I started researching sun sneezing

I thought that having the photic sneeze reflex

Must Convey some sort of evolutionary advantage under the people who have it. If you think about it

Sneezing is a way of transmitting disease. In your snot there can be the living Bacteria that cause

Tuberculosis, and strep throat or viruses that cause measles mumps Rubella and influenza, so if you're living in a moist dark cave

and you sneeze

Where your snot lands on the floor or the walls of the cave and those Pathogens can stay alive for hours or even days

increasing the Likelihood of spreading that disease to other people who live with you in your cave

Whereas if you only sneeze when you emerge from the darkness of the cave and into bright sunlight

Well, then that mucus will quickly dry out or because it's exposed to the harmful UV rays of the sun

It will kill all of those pathogens

Greatly decreasing the risk of spread of disease

By the 1960s some studies were revealing the hereditary nature of the photic sneeze reflex

There was a father who would sneeze two times when he entered bright sunlight

And when they tested his baby daughter who was just four weeks old

Moving her from a dark room into full sunlight also caused her to sneeze

Exactly two times and by the 1980s it was clear that this trait was

autosomal dominant

Meaning that you only needed to inherit one

Copy of the Gene from just one of your parents in order to exhibit the trait

But when studies the population have been done only about 18 to 35 percent of people actually have the photic sneeze reflex

So I don't really think this gives a great evolutionary advantage, otherwise everyone would have it

you know it could just be one of those random mutations that happens over time and

Sustains itself because it's neither good nor bad evolutionarily speaking

So it affects roughly one in four people

Can you spot the person with the photic sneeze reflex?

Nowadays, it's actually been found that the gene responsible for the photic sneeze reflex is on the second chromosome

And it's a single letter of DNA that's been changed, on my second chromosome

I have a C

Where non sun sneezers have a T, and the reason that we know this is that back in 2010 there was a study of about

10,000 people where they went online and they

Reported whether they were sun sneezers or not and then their DNA was

analyzed and the groups were compared and what they found was the thing the sun sneezers had in common was this one particular

letter change in their DNA.

That I think is pretty amazing.

Now it's unclear exactly how this change affects your physiology and makes you more susceptible to sun sneezes

But the best theory at the moment is that it involves the trigeminal nerve which is the largest cranial nerve it

Involves all of the feeling that you have in your face and as the name implies

It actually has three branches one of which receives stimuli from your eye and another which receives stimuli from your nose

so the thought is that this really active stimulation of the optic nerve may cross over into the

maxillary Branch causing that little tickle which gets you to sneeze

Photic Sneezes are generally pretty harmless unless you're doing something like flying a fighter jet or performing some sort of delicate surgery.

But this methodology,

can allow us to learn a lot about different heritable traits and diseases just by studying lots of people and their genotypes

This episode of veritasium was supported by 23andme. Which is actually the company that figured out that gene polymorphism

responsible for the photic Sneeze reflex

the name of the Company 23andme comes from the fact that humans actually have 23 pairs of chromosomes and

The purpose of the company is to give people insight into their own DNA

So what you do is spit into a vial

which is actually much harder than it sounds and

Send it off to them and of course in your saliva are some of the cells from your mouth

They analyze the DNA from those cells

And then they send you a report where you can find out tons of stuff about your genes like

Where do your genes come from that is what's your ancestry and do you have the photic sneeze reflex? Do we share the same?

Change of DNA on our second chromosome you can also learn about your health and other physical traits and how much

Neanderthal you are so if you want to learn more about your genotype

Then you should go to 23andme.com/veritasium, so I want to thank 23andme for supporting me

And, I want to thank you for watching.