Eye color is one of the most distinctive physical traits in humans. The color of your eyes depends on the amount and type of pigment in your iris. This pigment, called melanin, is produced by cells called melanocytes. The amount and type of melanin in your iris is primarily determined by genetics.
How Genes Determine Eye Color
Human eye color is controlled by multiple genes. The main genes involved are:
– HERC2 gene – Controls how much melanin is produced
– OCA2 gene – Determines whether melanin is brown or blue
– Other genes – Modify eye color slightly, creating shades of colors
The HERC2 gene contains the instructions for making a protein that regulates OCA2, which is the main gene that determines eye color. The HERC2 gene comes in two versions, or alleles:
– Herc2 B – Produces a lot of pigment
– Herc2 b – Produces less pigment
The OCA2 gene determines whether the melanin made is brown (eumelanin) or blue (pheomelanin). People with two copies of the OCA2 version that produces brown eyes (homozygous) will have brown eyes. People with two copies of the OCA2 version that produces blue eyes (homozygous) will have blue eyes.
So in summary, the HERC2 gene controls melanin production while the OCA2 gene determines melanin type. Different combinations of these two genes account for the various eye colors we see.
How Parents Pass on Eye Color Genes
We each inherit two copies of the HERC2 and OCA2 genes, one from each parent. The eye color genes a child inherits follows these rules:
– Parents pass on one of their two gene copies to their child randomly
– The child’s eye color depends on the specific combination of genes inherited
For example, if one parent has brown eyes (BB on OCA2, bb on HERC2) and the other parent has blue eyes (bb on OCA2, Bb on HERC2), here are the possible eye color combinations the child could inherit:
| Parent 1 | Parent 2 |
|---|---|
| BB on OCA2 | bb on OCA2 |
| bb on HERC2 | Bb on HERC2 |
| Possible Child Genotypes | Child’s Eye Color |
|---|---|
| Bb on OCA2, bb on HERC2 | Brown |
| Bb on OCA2, Bb on HERC2 | Brown |
| bb on OCA2, bb on HERC2 | Blue |
| bb on OCA2, Bb on HERC2 | Blue |
As you can see, the child has a 50/50 chance of having brown or blue eyes in this scenario. Their eye color depends on the random combination of genes inherited from each parent.
Exceptions and Modifications
In some cases, a genetic mutation can lead to eye colors outside of brown, blue, and common mixtures:
– Green eyes – Caused by an OCA2 mutation that produces less brown melanin
– Gray eyes – Result from a reduced amount of overall melanin production
– Hazel eyes – Caused by a combination of light brown, green and gold
– Amber eyes – The result of a yellow lipid layer in the iris
Additionally, the amount of melanin produced can be influenced by other genes. These genes interact with HERC2 and OCA2 to modify eye color slightly. Altogether, it’s estimated that at least 8 different genes impact eye color.
When Eye Color Is Determined
Eye color is determined before birth and can be seen in the fetus from around 3 months gestation. However, melanin levels may still increase in the first year after birth leading to subtle changes in eye color. At around 1 year old, the eye color a child is born with is set.
Predicting a Child’s Eye Color
It is tricky for parents to predict a child’s eye color based on their own. This is because multiple genes are involved and each parent passes down a random selection.
General guidelines for predicting eye color based on parental genes:
– If both parents have blue eyes, the child will most likely have blue eyes
– If one parent has blue eyes and the other brown eyes, the child has around a 50% chance of having blue eyes and 50% chance of brown
– If both parents have brown eyes, the child will most likely have brown eyes
– If one parent has green eyes, the child has a chance of inheriting green eyes
These are just probabilities however, not guarantees. The only way to know for sure is to have genetic testing done to identify the specific eye color genes present in each parent. Even then, there is a small chance of random mutation resulting in an unexpected eye color in the child.
Can Eye Color Change Later in Life?
Eye color is set by age 1 and cannot spontaneously change later in life. However, some factors can modify the appearance of eye color slightly over time:
– Iris pigment cells slow down over age 50, reducing melanin and making eyes appear lighter
– Eye trauma, disease, or buildup of deposits on the iris can change eye color
– Certain medications like Latanoprost eye drops can increase brown melanin
– Lasers resurface the iris and stimulate melanin production, darkening eye color
– Amount of melanoma declines with age, causing yellowing and lightening
But while these factors may create subtle changes in shade or luminosity, true eye color that’s linked to the ratio of melanin pigments remains the same throughout life.
Conclusion
In summary, eye color inheritance is a complex process determined mainly by two genes, HERC2 and OCA2. Each parent randomly passes on one copy of these genes to their child. The specific combination then produces the child’s eye color, which is usually set by age one. While certain factors can slightly alter the appearance of eye color later in life, a person’s true genetic eye color remains static. So in the end, both parents play an equal role in determining their child’s eye color.
