There's a Genetic Reason Why Some People Sneeze in The Sunshine

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.

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4 facial pains and what they mean

Facial pain, say experts, point to conditions that affect varied parts of the body

Twenty-three-year-old IT engineer,  made peace with recurrent headaches she had been battling for 10 years. She'd avoid brightly-lit and air-conditioned rooms, and when the pain got unbearable, she'd reach out to a painkiller. "That would sort me out for a few days, but it would be back in three days," she says.

A visit this year to pain specialist  however, led to the realisation that the source of the headache was in the muscles of her neck. Her posture was the culprit. She was diagnosed with the classic turtle-head posture-forward head and sloping shoulders.

The doctor, who says she receives 50 new cases a week at her clinics  says pain in the orofacial region (face, mouth, nose, ears, eyes, neck, and head) is usually the result of a problem somewhere far from it. Dr  says, "Patients often complain of chronic headaches at the top of the head along with stiffness at the base of the skull that radiates to the tips of the fingers. Often, this is wrongly attributed to stress."

Why does your face hurt

4 facial pains and what they could point to

A. Symptom: Deep, dull headache
Condition: Neck muscle spasms
Caused by: Whiplash (neck injury), poor posture and long hours at computer, physical stress

How does a spasm in the neck muscle play out as a headache, ? Medically termed as myofascial pain, this is common and characterised by discrete tender areas or trigger points. The spasm may occur in the centre of the neck muscle but it can play itself out at any point along the length and width of the muscle, like the top of the head. Therefore, a muscle spasm in the neck can show up as a debilitating headache, says Dr. "These deep, dull headaches are often referred to as cervical headaches and more commonly as 'tension headaches'. But the tension does not refer to psychological stress but actual tensile forces acting on the muscle, Dr. clarifies.

B. Symptom: Ache at base of skull, neck and face
Condition: Rounded shoulders (slouching)
Caused by: Poor sleeping posture (wrong pillow) and over-exercising upper body at the gym

The shoulder and the head share muscles that pass through the neck. Physiotherapist  explains: "The internally rotated (forward rotated) shoulders increase the compression on the collar bone and the attached muscles. There is a shortening of the pectoral (chest) and scapular (shoulder blade) muscles. As the head and neck are brought forward, the patient is forced to extend the back of the neck to keep the eyes horizontal, resulting in over-activity of the muscles at the base of the skull." When these muscles are tightened, the head and shoulders are pulled forward leading to Forward Head Posture and rounded or slouching shoulders. The constant effort to bring the head back into its normal position is what causes pain at the base of the skull, neck and face.

C. Symptom: Electric shock-like pain in cheeks, near nose and jaw
Condition: Trigeminal Neuralgia (TN)
Caused by: Blood vessel pressing on a nerve inside the skull, multiple sclerosis

 It's an impingement on a nerve branch in the spinal cord or brain stem that shows up as sharp, shooting, burning pain around the nose and above the lips. Neuro-surgeon  says, "The trigeminal nerve has three divisions that sub-serve sensations over 1) forehead, 2) cheeks, and 3) lower jaw. Though predominantly a sensory nerve, it also has a motor division that supplies the muscles that aid chewing. The nerve enters the brain stem in the trigeminal nucleus and from there it has projections higher up in the brain." All nerves are covered by a sheet called myelin, which works like an insulation around a wire, preventing leakage of current. The trigeminal nerve is myelinated, except in the last few millimeters where it enters the brain stem — here it remains naked, and ultrasensitive. It is this unmyelinated area where throbbing of a blood vessel makes it excitable, resulting in severe pain. This intermittent pain is triggered by even the slightest external stimulation — touch, brushing teeth, chewing, or even a draft of wind. "The pain is so severe, it can drive a person to contemplate suicide, says Dr.

D. Symptom: Ache in temples, ear pain, facial soreness, limited range of motion of mouth, clicking and joint noise
Condition: Jaw disorder
Caused by: Grinding of teeth in sleep, cradling phone between neck and shoulder, biting nails, clenching teeth during a task that requires focus

Temporomandibular Joint (TMJ) or the jaw joint is located in front of both ears and connects the lower jaw with the skull. The TMJ along with the muscles of mastication allow you to chew, yawn, shout, whistle, talk and sing. "Considering it is the most used joint in the body, a TMJ dysfunction can be severely debilitating," says Dr. "TMJ disorders can be felt as tooth pain even in the absence of dental pathology and can contribute to snoring and sleep apnoea," she adds.




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