Eye color is determined by genetics. The main gene that controls eye color is the OCA2 gene, which produces a protein involved in producing the pigment melanin. The amount and type of melanin in the iris of the eye determines eye color. Brown eyes contain a large amount of melanin, while blue eyes contain much less melanin.
Generally, when both parents have brown eyes, their children are likely to also have brown eyes. However, sometimes two brown-eyed parents can have a child with a different eye color, such as green, blue, or hazel eyes. This article will examine how this can happen genetically.
How eye color is inherited
Eye color is considered a polygenic genetic trait, meaning multiple genes influence the final eye color. The main gene involved is OCA2, but other genes also impact eye color. Each person has two copies of the OCA2 gene, one inherited from each parent. The OCA2 gene has different versions or alleles. The main alleles are:
- B – produces a large amount of brown melanin
- b – produces a small amount of brown melanin
- G – produces a small amount of green melanin
- g – produces a small amount of gray melanin
The B and b alleles are considered “dominant” over g and G alleles. This means if a person has a B or b allele, they will have brown eyes. The G allele leads to green or hazel eyes, while the g allele leads to blue/gray eyes.
Here are some examples of possible OCA2 gene combinations and the resulting eye colors:
- BB or Bb – Brown eyes
- bb – Brown, potentially lighter brown eyes
- BG or bG – Hazel or green eyes
- gg – Blue/gray eyes
So, for two brown-eyed parents to have a child with colored eyes, each parent must carry a non-brown allele in addition to a brown allele. If both parents carry the recessive g allele, their child could end up with gg and blue eyes. Or, if one parent has G and one has g, their child could inherit Gg and green/hazel eyes.
Other genes that impact eye color
In addition to OCA2, there are at least 8 other genes that help determine eye color:
- HERC2 – Turns on expression of OCA2
- SLC24A4 – Involved in melanin production
- TYR – Produces tyrosinase enzyme for melanin production
- SLC45A2 – Helper protein for tyrosinase
- IRF4 – Turns on expression of TYR
- OCA2 Ala481Thr – Found near OCA2, modifies function
- TYRP1 – Involved in melanin production
- ASIP – Inhibits melanin production
Variants in these genes interact with OCA2 to influence the final eye color. This further allows for the possibility of colored eyes being produced by two brown-eyed parents. For example, a parent could have brown eyes due to dominant OCA2 alleles but carry recessive variants in the HERC2 or SLC24A4 genes. If the child inherits these recessive variants, they could override the OCA2 brown alleles and result in lighter eyes.
Overall, the many genes involved in determining eye color allow for significant variation in inheritance and expression of this polygenic trait.
Can parents’ eye color change over time?
One question that sometimes comes up is whether a parent’s eye color changed or was originally incorrectly identified, rather than two brown-eyed parents genetically producing a child with colored eyes. While eye color is set at birth by genetics, in some cases, a person’s eye color can appear to change over the course of their life. Here are some reasons a person’s eye color may appear to change:
- Melanin levels decrease over time – This can make brown eyes appear lighter or more amber in later decades of life.
- Lasik surgery – Altering the surface of the cornea or thickness of the eye can cause eyes to take on a different hue.
- Increased limbal rings – These dark rings around the iris can make light eyes look darker or more green.
- Changes in lighting conditions – Eyes can look darker in low light and lighter in bright conditions.
- Eye trauma or disease – Injury or conditions like glaucoma or cataracts can also shift eye color.
However, the fundamental genetic makeup and eye color remains the same throughout life. Any changes are only superficial changes in how the eye absorbs and reflects light. So, while a parent’s eye color may appear slightly different over the years, it’s extremely rare for brown eyes to shift to a true green, blue, or hazel later in life.
What are the odds of brown-eyed parents having a child with colored eyes?
It’s challenging to determine exact odds of two brown-eyed parents having a child with colored eyes. Some research studies and estimates have suggested:
- About 1 in every 10 brown-eyed couples carry genetic markers for green eyes that could be passed on.
- Approximately 1 in 20 brown-eyed couples have a brown-eyed child born with green eyes.
- The odds of having a blue-eyed child when both parents have brown eyes is around 1 in 300.
However, many factors make it hard to pin down precise statistics. The numerous eye color genes make patterns of inheritance complex. Populations have different prevalence of the various eye color alleles. And limited historical data makes it hard to quantify exact probabilities over generations.
So while it is uncommon for two brown-eyed parents to have a child with colored eyes, it is certainly possible genetically. The odds increase if the brown-eyed parents have known ancestry or family members with light eyes. Genetic testing panels can also help provide more insight into a couple’s likelihood of having a child with eyes that differ from their own.
Examples of reported cases
While unusual, there are many reported examples of brown-eyed parents giving birth to children with colored eyes:
- In 2008, England’s The Telegraph reported on brown-eyed parents, Hayley and Charlie Ridge, who had a blue-eyed son, Harry.
- In 2015, a story about brown-eyed Nicole and Matt Griffin went viral when their daughter Aliyah was born with brilliant blue eyes.
- In 2018, INSIDER covered brown-eyed Michael and Juliana Ritter of Germany having a son, Mats, with piercing blue eyes.
Genetic testing later confirmed that these parents carried recessive variants linked to blue eyes. While extremely rare, the inherited combinations of the parents’ eye color genes resulted in children with blue eyes.
These cases and others demonstrate that, while uncommon, two brown-eyed parents can indeed have children born with green, blue, or hazel eyes.
Could a non-parental event lead to colored eyes?
Given the rarity of two brown-eyed parents having a child with colored eyes, questions can arise about the child’s paternity. However, non-parental inheritance is extremely rare. Some possibilities could include:
- Recessive genes from both parents: As described above, two brown-eyed parents could each carry recessive alleles that come together to produce colored eyes in their offspring.
- Chimerism: In very rare cases, chimerism can result in a person having two different sets of DNA. If one set carries genetic variants for light eyes, some of the person’s cells including their irises could express different genes than other cells.
- Genetic mosaicism: Post-zygotic mutations could alter eye color genetics in some cells in the body during early embryonic development.
- Medical conditions: In exceptional cases, certain conditions like Waardenburg syndrome can cause changes in melanin production and distribution that affect eye color.
However, in the vast majority of cases, when brown-eyed parents have a child with colored eyes, the explanation lies in the genetic inheritance from both parents. Accurate paternity should not be questioned based solely on this rare occurrence.
Conclusion
It is uncommon, but possible, for two brown-eyed parents to have a child born with colored eyes like green, blue, or hazel. This results from each parent carrying recessive variants in the OCA2, HERC2, SLC24A4 or other eye color genes. When these recessive gene versions are combined in a child, they can override the typical brown-eye alleles and lead to colored eyes. Though unusual, examples of this happening demonstrate that eye color genetics is complex and inheritance patterns are not always straightforward. So while kids often resemble their parents, sometimes genetics and chance can lead to unique attributes like colored eyes being expressed in the next generation.