Genetics of Eye Color (2024)

Genetics of Eye Color (1)By Sally Robertson, B.Sc.Reviewed by Kate Anderton, B.Sc. (Editor)

A person’s eye color depends on how much of a pigment called melanin is stored in the front layers of the iris, the structure surrounding the pupil. Specialized cells called melanocytes produce the melanin, which is stored in intracellular compartment called melanosomes. People have roughly the same number of melanocytes, but the amount of melanin within melanosomes and the number of melanosomes within melanocytes both vary.

Eye color ranges depending on how much melanin is stored in these compartments. In people with blue eyes a minimal amount of melanin is found within a small number of melanosomes. People with green eyes have a moderate amount of melanin and moderate number of melanosomes, while people with brown eyes have high amount of melanin stored within many melanosomes.

Genes involved in determining eye Color

The amount of melanin stored is determined by genes that are involved in the production, transport and storage of melanin.

To date, researchers have discovered more than 150 genes that influence eye color, a number of which have been discovered through studies of genetic disorders. Others have been identified during genomic studies of mice and fish.

Some genes play a major role in determining eye color, while others only have a small contribution.

One region of chromosome 15 contains two genes located near to each other that play major roles in determining eye color. One gene, called OCA2, codes for a protein called P protein, which is involved in melanosome maturation and affects the amount and quality of melanin stored in the iris. A number of genetic variations (polymorphisms) in this gene reduce how much P protein is produced and result in a lighter eye color.

The other main gene involved is called HERC2. Intron 86 on this gene controls the expression of OCA2, activating it or deactivating it as required. At least one polymorphism in this intron reduces the expression and activity of OCA2,which reduces how much P protein is produced.

A number of other genes play smaller roles in eye color. The roles of the genes ASIP, IRF4, SLC24A4, SLC24A5, SLC45A2, TPCN2, TYR, and TYRP1 are thought to combine with those of OCA2 and HERC2.

Eye color inheritance pattern

Due to the number of genes involved in eye color, the inheritance pattern is complex. Although a child’s eye color can generally be predicted by looking at the color of the parents’ eyes, the polymorphisms that can arise mean a child may well have an unexpected eye color.

A child’s eye color depends on the pairing of genes passed on from each parent, which is thought to involve at least three gene pairs. The two main gene pairs geneticists have focused on are EYCL1 (also called the gey gene) and EYCL3 (also called the bey2 gene).

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The different variants of genes are referred to as alleles. The gey gene has one allele that gives rise to green eyes and one allele that gives rise to blue eyes. The bey2 gene has one allele for brown eyes and one for blue eyes. The allele for brown eyes is the most dominant allele and is always dominant over the other two alleles and the allele for green eyes is always dominant over the allele for blue eyes, which is always recessive. This means parents who happen to have the same eye color can still produce a different eye color in their child.

For example, if two parents with brown eyes each passed on a pair of blue alleles to their offspring, then the child would be born with blue eyes. However, if one of the parents passed on a green allele, then the child would have green eyes and if a brown allele was present, then the child would have brown eyes irrespective of what the other three alleles were.

Chromosome 15 - Eye colour

However, this does not explain why two parents with blue eyes can have a child with brown eyes. It also does not explain how grey or hazel eyes arise. This is where modifier genes, other genes associated with eye color and mutations all come into picture, as they can all lead to variability in eye color. Scientists are still studying exactly how these other factors cause such variations.

Genetic conditions that affect the eyes

Several genetic conditions affect the eyes, with two examples being ocular albinism and oculocutaneous albinism.

In the case of ocular albinism, severely reduced pigmentation of the iris results in very light-colored eyes and vision problems. Oculocutaneous albinism also affects pigmentation of the iris, but the problem involves the skin and hair as well. People born with this condition tend to have very fair skin, white or almost white hair in addition to having very light-colored irises.

Both conditions are caused by mutations in genes that contribute to melanin production and storage.

The presence of genetic variants can also lead to a condition called heterochromia, where an affected individual has eyes that are different colors to each other.

Further Reading

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Genetics of Eye Color (2024)

FAQs

What is the genetics of eye color? ›

Eye color is determined by variations in a person's genes. Most of the genes associated with eye color are involved in the production, transport, or storage of a pigment called melanin. Eye color is directly related to the amount of melanin in the front layers of the iris.

What are the eye color gene combinations? ›

Both parents with brown eyes: 75% chance of baby with brown eyes, 18.8% chance of baby with green eyes, 6.3% chance of baby with blue eyes. Both parents with blue eyes: 99% chance of baby with blue eyes, 1% chance of baby with green eyes, 0% chance of baby with brown eyes.

How many copies do you have of the gene for eye color? ›

You get one copy of each gene from each parent. This means you have two copies of most of your genes. So you have two copies of each of your eye color genes.

Which parent dominates eye color? ›

Babies inherit equal eye color genetics from both parents — 50% from each. From here, genes mutate to produce what are called alleles. Alleles are alternative forms of a gene that, in this case, are responsible for giving your baby a certain eye color.

What color is the rarest eye color? ›

Gray: The Rarest Eye Color

With this change, gray now tops the list as the rarest eye color. Gray eyes may contain just enough melanin in the front layer to dim the blue wavelengths of light that are reflected back by the tissue of the eye. Dark gray eyes have a bit more melanin in the front layer than pale gray eyes.

What eye color is the strongest gene? ›

The allele for brown eyes is the most dominant allele and is always dominant over the other two alleles and the allele for green eyes is always dominant over the allele for blue eyes, which is always recessive.

Is black eye color rare? ›

While some people may appear to have irises that are black, they don't technically exist. People with black-colored eyes instead have very dark brown eyes that are almost indistinguishable from the pupil. In fact, brown eyes are even the most common eye color in newborn babies.

Can eye color skip 2 generations? ›

These alleles would be passed down and expressed in each generation. Blue eyes can in fact skip generations because recessive alleles will be passed down in each generation, but not necessarily expressed due to the masking effects of dominant alleles.

Why are green eyes so rare? ›

Only about 2 percent of people in the world have naturally green eyes. Green eyes are a genetic mutation that results in low levels of melanin, though more melanin than in blue eyes. Green eyes don't actually have any color.

Is there really just one gene that controls your eye color? ›

Although there are about 16 different genes responsible for eye color, it is mostly attributed to two adjacent genes on chromosome 15, hect domain and RCC1-like domain-containing protein 2 (HERC2) and ocular albinism (that is, oculocutaneous albinism II (OCA2)).

Which eye color is the most dominant? ›

Eye colour, or more correctly iris colour, is often used as an example for teaching Mendelian genetics, with brown being dominant and blue being recessive.

Can two brown eyes make blue? ›

Flexi Says: Two brown-eyed parents (if both are heterozygous) can have a blue-eyed baby. If both the parents have brown eyes, then there is generally a 25% chance for their child to have blue eyes. Because both the brown-eyed parents have a recessive blue-eye gene and can pass it to the next generation.

Which parent determines height? ›

How to predict how tall a child will be. According to Johns Hopkins Medicine, people may wish to try the following formula for predicting how tall a child will be: Measure the height of both biological parents. For male children, add 5 inches (in) to the father's height, add the mother's height, then divide by 2.

Do blue eyes mean inbreeding? ›

This is a persistent myth with no scientific basis. Blue eyes are a result of genetics, not inbreeding. The diversity of eye colors within a population is a natural outcome of genetic variation. Blue eyes are not a mutation but rather a variation in the expression of genes related to eye color.

Where do hazel eyes come from? ›

Hazel eyes are due to a combination of Rayleigh scattering and a moderate amount of melanin in the iris' anterior border layer. Hazel eyes often appear to shift in color from a brown to a green. Although hazel mostly consists of brown and green, the dominant color in the eye can either be brown/gold or green.

Can brown-eyed parents have a blue-eyed child? ›

Flexi Says: Two brown-eyed parents (if both are heterozygous) can have a blue-eyed baby. If both the parents have brown eyes, then there is generally a 25% chance for their child to have blue eyes. Because both the brown-eyed parents have a recessive blue-eye gene and can pass it to the next generation.

How do hazel eyes happen in genetics? ›

Most likely, hazel eyes simply have more melanin than green eyes but less than brown eyes. There are lots of ways to get this level of melanin genetically. It may be that hazel eyes are the result of genes different from GEY and BEY2.

Which parent determines hair color? ›

While dark-haired parents tend to have dark-haired kids, hair color genetics is quite complex. Your hair color depends in part on how many genetic variants you inherit from both of your parents. If you have brown hair, your shade of brown depends on how many light and dark markers you inherit from your mom and dad.

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