I was intrigued not only because I have green eyes myself, but because this particular genetic trait will factor very heavily into one of my upcoming Science Fiction Romance works.
So what's up with green eyes? In a nutshell...
Green Eyes Aren't Really Green
In actuality, all iris colors range from light brown to black. All other eye colors are actually just a trick of the light!
Green eyes are really light brown pigmentation of the stroma of the iris as a result of low to moderate levels of melanin, that diffuse the light on the irises to make them appear green, in much the same way that the sky looks blue because of light diffusion. This is known as Raleigh Scattering which has specific properties that effect eye color.
Green Eyes are Very Rare
What makes green eyes so unique is that only 2% of the total population tend to carry the genetics that result in the appearance of green irises, though in some populations the percentage runs higher because of the regional genet pool. Green eyes are most common in individuals with recent Celtic or Germanic ancestors. (I'm descended from Scottish, German, French and Irish ancestors, so my eye coloration probably came from one or more of these branches of my family.)
Green Eyes Tend to have Chameleon Properties
Because the green coloration is actually just an effect of diffused light, things such as light conditions, the colors of eye makeup or clothing worn or colors of the surrounding environment can compliment or contrast with the green shade, making it range anywhere from a near-blue to hazel to dark gray. But these aren't the only factors can change the hue of green eyes.
So can mood. Anger can dilate blood vessel and redden the whites of the eyes, which makes the eyes appear greener in contrast. Two emotions at the opposite end of the scale--happiness and grief--can dilate the pupils, also causing the green coloration to appear darker, or even more blue.
Surprisingly, weather and temperature can also change the color of green eyes by changing the ambient light, which also affects the light diffusion and the color of the eyes.
The Genetic Origins of Green Eyes
The two genes thought to have the most influence on eye color are OCA2 and HERC2, both located in Chromosome 15, however there is evidence that up to sixteen different genes could be responsible for eye color. The genetics that affect eye color are also believed to be related to freckling, hair and skin tones. In truth, this is a pretty basic explanation. The genetics involving eye color are much more complex. (For instance, blue eyes are actually thought to be the result of a mutation in the HERC2 gene.)
Where and when green eyes emerged in the human species is not known, but research has revealed they were prevalent in southern Siberia in the Bronze age. In modern times, they are most often seen in Northern and Central Europe, and can be found in Southern Europe and Western Asia, specifically among the Ashkenazi Jews of Israel.
The third novel in my SFR series (working title "Draxis" but official published title yet to be determined), brilliant green eyes are a trademark of the ancestral group that colonizes the planet. The genetics involving green eye color became dominant over time, along with a bronze skin tone and light blonde hair. This unusual pairing of lighter eye and hair color with darker skin pigmentation was the result of fixed genetics stamping the entire population with these traits.
If you're intrigued by the variants affecting green eye coloration and want to learn more, you might want to start with this article: Learn About the Origins of People with Green Eyes
Are you part of the rare genetic group who has green eyes? If so, where do you think those genetics came from? Have you read (or written) any books where the green eye color is dominant or important to your species?