Like many of us, Derek Muller, host of the YouTube channel Veritasium, has a condition known as a photic sneeze reflex. The condition affects roughly one in four of us, and makes us sneeze whenever we go from a darker room into the sunshine.

Research into the genetics underlying photic sneeze reflex has found that just one letter's difference in our DNA code can determine if someone is a Sun sneezer or not - but we still don't know why humans have evolved such a strange anomaly. 

The variation is known as a single nucleotide polymorphism, or SNP: a spot where our DNA code varies slightly. And the result can actually trigger the unique photic sneeze reflex, as Derek explains in the video below:

So what's going on here? Research back in 2010 showed that having a C instead of a T at the genetic marker known as rs10427255 increases your chances of being a sun sneezer by about 1.3 times.

Interestingly, scientists also found links between Sun sneezing and a gene associated with light-induced epileptic seizures. 

Thanks to earlier studies, we also know that the genetics of the photic sneeze reflex are 'autosomal dominant', which means you only need to inherit the relevant gene from one of your parents to become a Sun sneezer yourself.

That fact has given rise to a brilliantly constructed acronym for the condition: the Autosomal-Dominant Compelling Helio-Ophthalmic Outburst, or ACHOO.

But it's still not clear why some of our bodies have evolved to do become Sun sneezers in the first place.

The leading hypothesis centres around the trigeminal nerve in the brain, which receives stimuli from both our eyes and nose.

"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," Derek explains in the video above.

In other words, the bright light of the Sun excites the ophthalmic branch of the trigeminal nerve, and because the nervous signals are so tightly packed together, this somehow triggers a sneeze as a response along the maxillary branch connected to the nose.

Exactly how this plays out, scientists still aren't sure, but it's a question that has a long, long history – even Aristotle wondered about the photic sneeze reflex in ancient Greece.

Sun sneezing may not sound like the most pressing problem in the world, but as we learn more about the reasons behind it, it could also help us get a better picture of human genetics as a whole.

And it could also improve our understanding of associated conditions such as epilepsy.

"This methodology can allow us to learn a lot about different heritable traits and diseases, just by studying lots of people and their genotypes," says Derek.