Let's first address what determines eye color. The short answer is genes - plain and simple. The slightly more complicated answer refers to the three known genes that are factors in deciding a person's eye color. According to this source , "there are three known gene pairs that control eye color. The bey 2 gene on chromosome 15 contains a brown and blue allele. Also on chromosome 15, the bey 1 gene is the central brown gene. On chromosome pair 19 the gey gene contains a green allele and a blue allele." Even more details about these chromosomes can be found here.
As you may have learned back in ninth grade biology, dark colors are dominant in eyes. Brown is dominant while green and blue are recessive, and green is 'more-dominant' than blue. The same source goes on to say, "A green eyed person would have a green allele on chromosome 19 and all or some other blue alleles. Blue eyes are produced only with two blue eye genes. All four alleles must be blue to produce a blue eyed person."
While much is known in this field, many questions remain a mystery. This source tells us that the molecules and proteins that make up the genes determining our eye color are still unknown. It even suggests that there may be yet another gene devoted to green eyes, as many eye colors are not easy to explain through the three chromosomes discussed above.
Photo courtesy of this source
that we know what determines eye color, let's talk about what makes it appear a certain way. The patterns within our eyes that make up each unique
pattern are decided by melanin.
Melanin is the same pigment used to make up hair, skin, etc. Our genes are the instructions for the
enzymes in our bodies. They signal
how much melanin should be layered in each iris. Darker eyes have more melanin, and lighter eyes have less
melanin, just as one would expect.
This source explains the layering of melanin in the iris to create each
eye color. There are two parts,
front and back, to the iris. For
blue eyes, no melanin is in the front and brown pigment is in the back so when
the light reflects and diffracts, it appears blue. For green eyes, there is a little pigment in the front and
the back. For brown eyes, there is
a lot of pigment on the front layer of the iris. Mixed eye colors (hazels, green/blue mixes, aka my eyes!)
are made from the melanin pigments being distribute in different patterns and
amounts among the layers (which is less straight forward than the descriptions
Photo courtesy of The Eye Si(gh)t
Lastly, why do eyes change colors?
According to All About Vision, eyes change color when the pupil changes in size (as a reaction to change in light) because the iris is either expanding or compressing with the pupil. The site also mentions that eye color can change with age, but it doesn't specify why that happens.
This brief New York Times article says that eyes can indeed change with age. It mentions a study on twins and how 10-15% of the identical twins saw changes in eye color, at the same rate, with age.
source I read through about eye color changing on a day-to-day basis doesn't
seem legitimate. Most sources have
been legends or suppositions that a person's mood is affecting this
change. I have yet to find a
source that scientifically backs up this thought.
Can you help me out by commenting to clear this up? Can eye color change with mood? The only common conclusions sites have given me is that mood and environmental influences only change the angle and amount of light, making the iris appear different, but not truly changing. Does this seem right to you?