My genetics are interesting to say the least. I have naturally black hair, yet my body hair is blonde. I have skin so pale my foundation is called snow, yet both my parents tan without struggle. My eyes also change six different colors: green, hazel, brown, blue, grey, and yellow. I had always attributed this to being a co-dominance gene; given each parent has color changing eyes as well. However, I never contemplated that there could be a different reason as to why.
A codominance gene is where both sets of genetics are equally as strong and neither hides the other; so in short they blend together. I was always told given that each of my parents have color changing eyes, and I got each of their colors, I had a codominance gene. Which seems logical enough.
However, an article from All About Vision gave me an answer I wasn’t too happy with. It says that human eye color is derived from three genes; which are accountable for brown, blue, and green eyes. However, colors such as grey and hazel (and of course combination color eyes) can’t fully be explained at this time. The article also said that a person’s eye color is never a mix of their parents. For example, a parent with blue eyes and a parent with green eyes won’t have a baby with a mixture of the two colors. Each parent has two sets of genes, and the child has a certain chance of getting any of the four genetic options.
The only possible explanation the article offers is pupil contractions and expansions. The pupil expands and contracts as a physical reaction to different levels of light. As well, your iris can expand as a physical reaction to certain emotions. This change in pupil size can change your eye color slightly; which explains why people often claim their eyes change color depending on what emotion they’re feeling.
Another article gives several other reasons why your eyes could appear to or do change color, as well as delving into hazel eyes a bit more. Besides the same answers of lighting and emotions, the article even mentions how you dress as a reason for a change of eye color. This would make a lot of sense given that certain eye colors are supposed to wear certain types of eye makeup and so on. This I have seen first hand with myself and several other people with hazel/multicolored eyes. Eye makeup meant for somebody with green eyes will make my eyes green, the same with blue, and so on.
The article also talks briefly about hazel eyes; which is what my license says my eye color is. Hazel eyes have always been believed to change color. They are also much more reflective than any other eye color. Which essentially means it could take on the color of something in my surroundings.
In a third article, they discuss the claim that eyes change color as you age. In short, it doesn’t. Once your eye color has fully matured, it does not not. They explain that it either has to be genetics or injury that causes change in eye color. Heterochromia, or two different colored eyes (think David Bowie or the scene from X-Men first class where Xavier hits on the blonde) are typically caused by traumatic injury. As a child, I hit my head on a ceramic tile kitchen floor and broke my nose. However, mine eyes are only sometimes two completely different colors.
Luckily there are several types of heterochromia . The one that seems to be closest to what I have is central heterochromia. Which means the outside of my iris is different from the inside. My outside is usually consistently blue or grey, with the inside seeming to change. Besides injury, heterochromia is also linked to several diseases; including diabetes, which I have.
So the conclusion? Well we have several plausible hypotheses. The answers that are most likely seems to be pupil contractions or central heterochromia; maybe even a mix of the two. However, it seems that we don’t have an exact answer.