Blue eyes are a recessive genetic trait caused by a lack of melanin pigment in the iris. For an individual to have blue eyes, they must inherit two copies of the recessive gene, one from each parent. The level of dominance of different eye colors is determined by genetics. Eye color is controlled by multiple genes, but the main gene implicated is the OCA2 gene which produces the P protein involved in melanin production. Different versions (alleles) of the OCA2 gene result in varying amounts of melanin pigment. The allele for brown/black eyes produces a lot of melanin while the allele for blue eyes produces very little. This makes brown/black eye color dominant over blue.
How Eye Color Is Inherited
Human eye color is a polygenic phenotypic character determined by two distinct factors:
- The pigmentation of the eye’s iris
- The frequency-dependence of the scattering of light by the turbid medium in the stroma of the iris
In simpler terms, eye color inheritance depends on:
- The amount and type of melanin pigment in the iris
- How the iris scatters light and reflects it back
The melanin pigment in the iris comes in two forms:
- Eumelanin – Brown/black melanin
- Pheomelanin – Red/yellow melanin
The amount of these melanins present determines eye color. Brown eyes have a lot of eumelanin, while blue eyes have very little of both eumelanin and pheomelanin. Green and hazel eyes have moderate amounts of eumelanin with more pheomelanin mixed in.
The main gene that controls melanin production in the iris is the OCA2 gene which produces the P protein. Different versions (alleles) of the OCA2 gene result in different amounts of melanin:
- The allele for brown/black eyes produces a lot of P protein and melanin
- The allele for green/hazel eyes produces a moderate amount of P protein and melanin
- The allele for blue eyes produces very little P protein and melanin
Dominance Hierarchy of Eye Colors
The various eye colors have a dominance hierarchy based on their melanin content:
| Eye Color | Melanin Content | Degree of Dominance |
|---|---|---|
| Brown/Black | High eumelanin | Most dominant |
| Green/Hazel | Moderate eumelanin + pheomelanin | Intermediate dominance |
| Blue | Low melanin overall | Most recessive |
As the table shows, brown/black eye color has the highest melanin content and is the most dominant. Blue eyes have the lowest melanin and are the most recessive. This means:
- Brown/black eyes are dominant over green/hazel and blue eyes
- Green/hazel eyes are dominant over blue eyes
- Blue eyes are recessive to both brown/black and green/hazel eyes
This hierarchy of dominance exists because the OCA2 alleles follow a similar hierarchy – the alleles for moderate to high melanin production are dominant over the allele for low melanin production that results in blue eyes.
What Eye Colors are Blue Eyes Recessive To?
Blue eyes are recessive to both brown/black eyes and green/hazel eyes. This means:
- If a brown/black eyed parent has a child with a blue eyed parent, the child most likely will have brown/black eyes.
- If a green/hazel eyed parent has a child with a blue eyed parent, the child most likely will have green/hazel eyes.
For a child to have blue eyes, both parents must carry the recessive allele for low melanin production. The examples below illustrate this:
Brown/Black Eyes are Dominant over Blue Eyes
| Parent 1 | Parent 2 | Child’s Genotype | Child’s Eye Color |
|---|---|---|---|
| BB | BB | BB | Brown/Black |
| BB | bb | Bb | Brown/Black |
| bb | bb | bb | Blue |
Where:
- B = Dominant allele for brown/black eyes
- b = Recessive allele for blue eyes
As seen, blue eyes result only when both parents pass on the recessive b allele.
Green/Hazel Eyes are Dominant over Blue Eyes
| Parent 1 | Parent 2 | Child’s Genotype | Child’s Eye Color |
|---|---|---|---|
| GG | GG | GG | Green/Hazel |
| GG | bb | Gb | Green/Hazel |
| bb | bb | bb | Blue |
Where:
- G = Dominant allele for green/hazel eyes
- b = Recessive allele for blue eyes
Again, blue eyes only result when both recessive b alleles are inherited.
So in summary, blue eyes are recessive to both brown/black eyes and green/hazel eyes due to the low melanin content.
Real World Examples
We can look at some real world examples of how blue eyes are recessive:
- Prince William has blue eyes while his wife Kate Middleton has brown eyes. Their children Prince George, Princess Charlotte and Prince Louis all have blue eyes. This is because even though Kate carries the dominant brown eye allele, she must also carry the recessive blue eye allele which was passed down to their children who inherited two copies of the blue eye gene.
- Actress Scarlett Johansson has green eyes while her twin brother Hunter has blue eyes. Since they share the same genetic makeup, this means their parents carried both the dominant green eye allele and recessive blue eye allele which segregated to the children.
- Former spouses Kim Kardashian (brown eyes) and Kris Humphries (blue eyes) have a daughter North West who has brown eyes. She inherited one dominant brown eye allele from Kim which overrode the recessive blue eye allele from Kris.
These examples reflect how blue eyes are recessive in the real world. Children can only have blue eyes if both parents carry and pass on the allele for low melanin production.
Determining Probability of Eye Color
We can use Punnett squares to determine the probability that a child will have blue eyes based on their parent’s genotypes:
If one parent has brown eyes (BB) and one has blue eyes (bb)
| B (Brown eyes) | b (Blue eyes) | |
|---|---|---|
| B (Brown eyes) | BB | Bb |
| b (Blue eyes) | Bb | bb |
There is a 0% probability the child will have blue eyes.
If one parent has green eyes (GG) and one has blue eyes (bb)
| G (Green eyes) | b (Blue eyes) | |
|---|---|---|
| G (Green eyes) | GG | Gb |
| b (Blue eyes) | Gb | bb |
There is a 0% probability the child will have blue eyes.
If both parents have blue eyes (bb x bb)
| b (Blue eyes) | b (Blue eyes) | |
|---|---|---|
| b (Blue eyes) | bb | bb |
There is a 100% probability the child will have blue eyes.
So two blue eyed parents are needed to guarantee blue eyes in offspring. A blue eyed parent paired with brown/black or green/hazel eyed parent results in 0% chance of blue eyes.
Can Two Brown-Eyed Parents Have a Blue-Eyed Child?
It is extremely rare for two brown-eyed parents to have a blue-eyed child, but it can happen if both parents carry the recessive blue eye allele.
For example:
| Parent 1 | Parent 2 |
|---|---|
| Bb | Bb |
In this case, both parents are brown eyed since the dominant B allele masks the recessive b allele. But since both carry the recessive b allele, there is a 25% chance they could have a bb child with blue eyes.
However, the vast majority of the time, two brown-eyed parents will only have brown-eyed children. The only exception is if they both coincidentally carry genetic mutations leading to reduced melanin production. But this scenario is exceedingly rare.
Conclusion
In summary, blue eyes are recessive to both brown/black eyes and green/hazel eyes due to their low melanin pigment levels. For an individual to have blue eyes, they must inherit two copies of the allele for low melanin production, one from each parent. Otherwise, the presence of a dominant brown or green eye allele from one parent is enough to confer those eye colors over blue eyes. Therefore, it takes two blue-eyed parents to have a 100% chance of passing on blue eyes. Understanding the genetic dominance patterns of eye color helps explain how this polygenic trait is inherited.
