The photic sneeze reflex is what causes many people to sneeze when when suddenly exposed to bright light, for example walking out of a dark movie theater. A third of the population seems to experience this odd phenomenon but scientists don't yet know much about the genetics behind it. From Scientific American:
The genetic culprit remains unidentified, but scientists are starting to take an interest in trying to find out. "I think it's worth doing," says Louis Ptácek, a neurologist at the University of California, San Francisco, and an investigator at the Howard Hughes Medical Institute. Ptácek studies episodic disorders such as epilepsy and migraine headaches, and he believes that investigating the photic sneeze reflex could shed light on their related neurology.
Epileptic seizures are sometimes triggered by flashing lights and migraine headaches are often accompanied by photophobia. "If we could find a gene that causes photic sneezing, we could study that gene and we might learn something about the visual pathway and some of these other reflex phenomena," Ptácek says.