The color of a person’s eyes depends on the amount and distribution of melanin in the iris. Melanin is a pigment that gives color to skin, hair, and eyes. People with low amounts of melanin in the iris tend to have blue, green, or grey eyes. Those with higher amounts of melanin have brown, amber, or hazel eyes. So if your eyes appear bluish grey, it means you have low to moderate levels of melanin in your irises. There are a few key factors that determine how much melanin each person has and leads to variations in eye color:
Genetics and Inheritance
Genetics plays a major role in eye color. The main gene that controls melanin production is the OCA2 gene located on chromosome 15. Different versions (alleles) of this gene are associated with more or less melanin. The Heritability of eye color, or the proportion of variation in a population due to genetic factors, is estimated to be as high as 98%. This means inheriting certain alleles from your parents greatly shapes the eye color you end up with.
Here is a simple overview of some common eye color genetics:
| Parental Genotypes | Possible Offspring Eye Colors |
|---|---|
| BB and BB | Blue |
| BB and Bb | Blue or Green/Hazel |
| Bb and Bb | Blue, Green/Hazel, or Brown |
| bb and bb | Brown |
Where B = OCA2 allele associated with high melanin and b = OCA2 allele associated with low melanin. As you can see, inheriting different combinations of melanin-producing alleles from your parents leads to variation in eye color.
Melanin Amount and Tyrosine
The main pigment responsible for eye color is eumelanin, a brown-black melanin. Higher amounts of eumelanin lead to brown/dark eyes while lower amounts lead to blue/light eyes. Eumelanin is synthesized from the amino acid tyrosine. Therefore, the amount of tyrosine present affects the production of melanin pigments.
Factors like your genetics, diet, and health conditions can affect tyrosine levels and thus how much melanin pigment your body produces. Deficiencies in amino acid intake, iron, copper, and vitamins B6 and C may lower tyrosine and melanin levels. Medical conditions affecting melanin synthesis like oculocutaneous albinism can also drastically reduce melanin and lead to much lighter eye colors.
Melanin Distribution and Density
It’s not just the total amount of melanin, but also the distribution and density of melanin granules within your irises that affects eye color. Eyes with a high density of melanin concentrated at the front of the iris tend to appear dark brown. Lighter eyes have fewer melanin granules that are more diffusely spread across the iris.
The melanin density of your irises is set before birth. However, melanin amount and distribution can change slightly over your lifetime. This is why some people’s eyes get darker with age as melanin content increases. The change is often more noticeable in light-colored eyes like blue and green.
Rayleigh Scattering and Stroma
For people with lower melanin levels, another factor influencing eye color is Rayleigh scattering. This is the scattering of light by particles much smaller than the wavelengths of radiation.
The stroma is the thin, fibrous part of the iris that lies in front of the pigmented epithelium. In eyes with low melanin, the stroma acts as the main scatterer of light. Shorter blue wavelengths are scattered more readily than longer red and green wavelengths. This makes the stroma and overall eye color appear more blue.
Eyes with a darker blue/grey appearance have a bit more melanin than light blue eyes. The additional melanin absorbs more blue light, shifting the color toward grey. Differences in stroma thickness and density also affect how much blue scattering occurs.
Amount of Lipochrome
In addition to melanin, another pigment called lipochrome (also known as carotenoid) is deposited in the iris during development. Lipochrome creates flecks of gold, yellow, and orange color in the eyes. This is why you may notice gold-ish spots or rings in blue, green, and hazel eyes. Higher amounts of lipochrome lead to greener and hazel eyes.
Multiple Genes Interacting
Though OCA2 is the main gene affecting eye color, researchers have identified over a dozen other genes involved as well. This includes genes like SLC24A4, TYR, SLC45A2, and HERC2 which help regulate melanin production and deposition. The interaction of multiple genes helps explain the wide continuum of eye colors.
Rarely, a genetic mutation in a single gene can have a large effect and cause unusual eye colors like violet-blue. But in most people, it is the subtle polygenic effects of many genes working together that determines your eye shade. Overall, this complex genetics is what leads to the many shades of blue, gray, green, and hazel eyes.
Conclusion
In summary, bluish gray eyes result from having a medium-low amount of melanin pigment in your irises. Your genetics regulate how much melanin you produce through inheritance of different versions of genes like OCA2. Melanin amount is also affected by biological factors influencing melanin synthesis, like tyrosine levels.
In addition to total melanin content, distribution and density of melanin granules in the iris stroma generates blue, gray, and green hues through Rayleigh scattering of light. Variation in lipochrome levels also adds flecks of color to make eyes appear more blue-green or hazel. Ultimately, it is the interaction between dozens of melanin, lipochrome, and eye development genes that leads to the spectrum of human eye colors.
So in conclusion, if you have bluish gray eyes, you have inherited a mix of genetic variants leading to reduced, yet still moderately present, melanin levels in your irises. This gives your eyes a lighter blue/gray appearance based on the levels and distribution of melanin and lipochrome pigments.
| Keyword | Root Keyword |
|---|---|
| Why are my eyes bluish grey? | eye color |
