I met my husband in college. We lived in Florida, aptly nicknamed “The Sunshine State” due to the abundance of sunny weather. Given that fact, it didn’t take me long to notice that he had a strange reaction when exposed to the sun: he sneezed—not once or twice, but nearly every time he stepped outside on a bright day. He could even look at a bright artificial light to trigger a sneeze if his nose was tickling after sundown. For years I thought it was something special about him. It’s not. In fact, studies have shown that between 10 and 35% of the population sneeze when exposed to the sun or bright light. What causes this so-called sun sneezing?
What is Sun Sneezing?
The scientific term for sun sneezing is Photic Sneeze Reflex, or PSR. Believe it or not, people have been wondering what causes PSR since the fourth century B.C. In his Book of Problems, the Greek philosopher Aristotle wondered, “Why does the heat of the sun provoke sneezing?”. While scientists still aren’t sure exactly what causes the phenomenon, they are sure it has nothing to do with the heat of the sun. Instead, current models predict that PSR is due to the arrangement of the nerves in the eyes and nose and the way that those nerves are triggered in some people upon looking at a bright light. This video does a great job at explaining what scientists believe happens to cause a sun sneeze.
The Genetics of Sun Sneezing
The video above mentions another name for sun sneezing: the ACHOO syndrome. ACHOO stands for Autosomal Dominant Compelling Helio-Ophthalmic Outburst Syndrome. The important take-away from the acronym is “autosomal dominant”. What that means is that sun sneezing is genetic: it is passed on from parent to offspring in the DNA. Not only that, but it is a dominant trait. What does that mean?
You likely know that our observable traits (eye color, height, skin color, etc.) are determined by our DNA—specifically our genes. Many genes come in different “flavors” called alleles. Oftentimes, when more than one allele exists for a given gene, one allele will be dominant. If a person has a dominant allele for a gene, the dominant allele will be the one that is expressed. If the dominant allele is present, expression of other alleles for the gene (called recessive alleles) will be hidden.
Oftentimes, when we discuss genetics, we assign letters to the different alleles for a gene. The dominant allele is designated with a capital letter, while the recessive allele is assigned the lowercase letter. For the gene responsible for PSR, let’s assign the dominant allele that leads to sun sneezing a capital S, and the recessive allele we will denote s.
During conception, each of us received a copy of every gene from our parents: one from mom and one from dad. Consequently, we have two copies of each gene*. Because the allele for sun sneezing is a dominant allele, you only had to inherit one copy of the allele during conception in order to be a sun sneezer. Only individuals who carry two copies of the recessive allele will not have PSR.
I am not a sun sneezer. Given that fact, I know I don’t carry even a single copy of the sun sneezer allele. My husband is a sun sneezer. This means he must have at least one copy of the dominant PSR allele. But does he have one copy of the dominant allele or two?
He only has one. How do I know? No, I didn’t have his DNA tested with a commercial kit. I just used what I know about genetics to figure it out. You see, my older son is a sun sneezer while my younger son is not. The only way for our younger son to not be a sun sneezer is for him to have two copies of the recessive allele. I know that he inherited the recessive allele from me, because that’s the only allele I had to pass down. The other copy of the recessive allele had to have come from his father. Therefore, my husband has one copy of the dominant allele and one copy of the recessive allele.
You may remember Punnett Squares from your Biology class. Here, I’ve made two Punnett Squares in order to demonstrate the alleles my husband carries for the gene responsible for PSR. The Punnett Square on the left assumes that my husband has two copies of the dominant S allele. If that were the case, all of our children would have been born sun sneezers.
The Punnett Square on the right assumes that my husband has a copy of both the dominant and recessive alleles. In this scenario, we see that every time we have a child, there is a 50% chance that the child will be a sun sneezer and a 50% chance that he won’t. That’s exactly what happened! With an understanding of genetics and heredity, we can use our observable traits to work backwards to determine what genes we have. (Incidentally, as seen in the second Punnett Square, we can know that my older son has the same alleles as his father while my younger son has the same alleles as I do).
What about you? Are you a sun sneezer? Are there sun sneezers in your family? If so, perhaps you can use what you know to “peek inside” the genes of your family to determine what alleles you carry. There are many traits you can do this with (handedness, the ability to taste PTC, whether or not you can roll your tongue, and many more).
Subscribers to my Science Simplified newsletter will receive a free Personal Genetic Trait Survey in the coming weeks that they can use to explore their genetics. And if you or your students would like to learn more about the fascinating topic of genetics, you may be interested in my course Genetics and Heredity. In it, students learn the difference between genotype and phenotype and the principles which govern Mendelian and non-Mendelian Inheritance. Learn about the genetics of human blood types, sex-linked traits, why we only inherit mitochondrial DNA from our mothers, and so much more! To learn more, click here.
*This isn’t the case for genes carried on the X chromosome. Males only receive genes carried on the X chromosome from their mother. As a result, males only have one copy of genes carried on the X chromosome. This is the basis for sex-linked traits.