Pigments Behind Your Eye Color
People have varying eye colors and shades– from dark to light brown, green, hazel, black, gray, and blue.
In fact, a few people have different colors in both their eyes.
Despite these varying eye colors, you will be surprised to know that there are just two pigments in our eyes - brown and red.
The colored part of the eye is called the iris. The iris contains pigment-forming cells called melanocytes, the same ones present in our skin.
These melanocytes produce two pigments – eumelanin (which is brown-black) and pheomelanin (which is red).
The varying shades of eye color depend upon the amount of pigment produced.
For example, people with dark eye color have a large amount of brown-black eumelanin, whereas those with light blue eyes have very little pigment.
Image: Is Eye Color Genetic: Types of Melanin Pigment
Is Eye Color Genetic? - The OCA2 and HERC2 Genes
Just like fingerprints, eye color is unique to an individual and is genetically determined.
A genome-wide association study identified 115 genetic variants associated with eye color.
Two genes located close to each other on chromosome 15– OCA2 and HERC2 are said to play an important role in eye color determination.
Though the OAC2 gene influences 75% of eye color, other genes also play a role in melanin production.
The OAC2 gene gives instructions to produce P protein that helps in the maturation of melanosomes (cellular structures that make and store melanin).
The P protein influences the amount of melanin in the iris.
Common changes (called variations or polymorphisms) in the OAC2 gene may reduce the amount of P protein produced.
People with less P protein have less melanin in their eyes.
This also means they may have blue eyes instead of brown.
The HERC2 gene is located very close to the OAC2 gene.
A part of the HERC2 gene is also called intron 86. This region regulates the activity of the OAC2 gene.
At least one polymorphism in the HERC2 gene area has been shown to reduce the OAC2 gene and decrease P protein production.
This results in less melanin in the iris and light-colored eyes.
Inheritance of Eye Color
Though eye color is genetically-determined, it is also an inherited trait.
For a long, scientists believed that a single gene was responsible for an individual’s eye color. However, the inheritance of eye color is far more complex.
The study of inheritance and genetics of eye color are still in the nascent stages.
Image: Is Eye Color Genetic: Inheritance of Eye Color
Source: https://www.nature.com/articles/s41433-021-01749-x
Can Eye Color Be Predicted?
It is impossible to predict a baby's eye color with 100% certainty.
But with certain genetic rules, it is possible to make a fair guess.
For example, the brown eye is a dominant trait - this means it can hide traits of green and blue eyes.
To find the possibility of recessive traits, it's helpful to know the grandparents' eye colors.
For example, if the mother has blue eyes and her entire family has blue eyes and the father has brown eyes with his mother and father with brown and blue eyes, the kid will have a 50/50 chance of having a blue-eyed or brown-eyed child.
| Probability of Eye Color | ||||
| Parent 1 | Parent 2 | Blue | Green | Brown |
| Blue | Blue | 99% | 1% | 0% |
| Blue | Green | 50% | 50% | 0% |
| Blue | Brown | 50% | 0% | 50% |
| Green | Green | 25% | 75% | 0% |
| Green | Brown | 12% | 38% | 50% |
| Brown | Brown | 19% | 7% | 75% |
Source: https://www.verywellhealth.com/genetics-of-eye-color-3421603
Summary
- Melanocytes produce two types of pigments in the eye – eumelanin (brown-black) and pheomelanin (red).
- The greater the amount of pigment, the darker the eye color.
- Two genes – OAC2 and HERC2, play a vital role in eye color determination.
- The OAC2 gene determines 75% of eye color, while other genes influence the rest.
- Eye color inheritance is complex as several genes are involved.
- Though parents' eye color can help determine that of the child, some variations may cause unexpected results.
References
https://medlineplus.gov/genetics/understanding/traits/eyecolor/
https://www.verywellhealth.com/genetics-of-eye-color-3421603
