which eye color is dominant?

The vast majority of people in the world have brown eyes.

The second most common color is blue, but people can also have green, gray, amber, or red eyes.

Some people have eyes that are different colors than each other.

According to estimates, 70–79% of the world’s population have brown eyes, making it the most common eye color worldwide.

In fact, the American Academy of Ophthalmology (AAO) state that everyone on Earth had brown eyes around 10,000 years ago.

Nowadays, the AAO note that about half of those living in the United States, and a higher proportion of people in Africa and Asia, have brown eyes.

People with brown eyes are less likely to develop eye cancer, macular degeneration, and diabetic retinopathy than those with lighter colored eyes.

Brown-eyed people are, however, more at risk of cataracts as they get older.

Blue is the second most common eye color globally, with estimates suggesting that 8–10% of people have blue eyes.

In the U.S., that proportion is higher, at about 27%.

Scientists believe that it is possible to trace all blue-eyed people back to a common ancestor, who likely had a genetic mutation that reduced the amount of melanin in the iris.

Most people with blue eyes are of European descent.

Approximately 5% of the world’s population and 18% of people in the U.S. have hazel eyes, which are a mixture of green, orange, and gold.

Hazel eyes are more common in North Africa, the Middle East, and Brazil, as well as in people of Spanish heritage.

Amber eyes, which have slightly more melanin than hazel eyes but not as much as brown eyes, account for about 5% of the world’s population.

People of Asian, Spanish, South American, and South African descent are most likely to have amber eyes.

An estimated 2% of the world’s population have green eyes, making them very rare overall.

However, green eyes are very common in some parts of the world, including Ireland and Scotland.

In the U.S., where many people descend from ancestors from Ireland and Scotland, about 9% of people have green eyes.

Close to 3% of the world’s population have gray eyes.

People with gray eyes have little or no melanin in their irises, but they have more collagen in a part of the eye called the stroma.

The light scatters off the collagen in a way that makes the eyes appear gray.

People with albinism or ocular albinism usually have little or no melanin in the iris. This lack of pigment causes red or violet eyes.

As eye pigmentation is important for vision, people with ocular albinism often have problems with their eyesight.

A person with ocular albinism may have very blurry vision or poor depth perception. They may experience rapid, involuntary eye movements, have higher light sensitivity, or find that their eyes look in two different directions.

Heterochromia — in which a person has more than one eye color — affects less than 1% of people.

The two eyes might be completely different from one another, or one part of the iris might be different than the rest.

Countless students have been taught that a single gene controls eye color, with the allele for brown eyes being dominant over blue. Scientists now realize such a model is overly simplistic and incorrect.

What you need to know:

Introduction In 1907, Charles and Gertrude Davenport developed a model for the genetics of eye color. They suggested that brown eye color is always dominant over blue eye color. This would mean that two blue-eyed parents would always produce blue-eyed children, never ones with brown eyes.

For most of the past 100 years, this version of eye color genetics has been taught in classrooms around the world. It’s one of the few genetic concepts that adults often recall from their high school or college biology classes. Unfortunately, this model is overly simplistic and incorrect – eye color is actually controlled by several genes. Additionally, many of the genes involved in eye color also influence skin and hair tones. In this edition of Biotech Basics, we’ll explore the science behind pigmentation and discuss the genetics of eye color. In a future edition, we’ll discuss genetic factors that contribute to skin and hair color.

A primer on pigmentation The color of human eyes, skin and hair is primarily controlled by the amount and type of a pigment called melanin. Specialized cells known as melanocytes produce the melanin, storing it in intracellular compartments known as melanosomes. The overall number of melanocytes is roughly equivalent for all people, however the level of melanin inside each melanosome and the number of melanosomes inside a melanocyte varies. The total amount of melanin is what determines the range of hair, eye and skin colors.

There are a number of genes involved in the production, processing and transport of melanin. Some genes play major roles while others contribute only slightly. To date, scientists have identified over 150 different genes that influence skin, hair and eye pigmentation (an updated list is available at http://www.espcr.org/micemut/). A number of these genes have been identified from studying genetic disorders in humans. Others were discovered through comparative genomic studies of coat color in mice and pigmentation patterns in fish. (A previous Biotech101 article that provides an overview of comparative genomics can be found here.)                                                   figure one

Eye color genes In humans, eye color is determined by the amount of light that reflects off the iris, a muscular structure that controls how much light enters the eye. The range in eye color, from blue to hazel to brown (see figure one), depends on the level of melanin pigment stored in the melanosome “packets” in the melanocytes of the iris. Blue eyes contain minimal amounts of pigment within a small number of melanosomes. Irises from green–hazel eyes show moderate pigment levels and melanosome number, while brown eyes are the result of high melanin levels stored across many melanosomes (see figure two, left).

To date, eight genes have been identified which impact eye color. The OCA2 gene, located on chromosome 15, appears to play a major role in controlling the brown/blue color spectrum. OCA2 produces a protein called P-protein that is involved in the formation and processing of melanin. Individuals with OCA2 mutations that prevent P-protein from being produced are born with a form of albinism. These individuals have very light colored hair, eyes and skin. Non-disease-causing OCA2 variants (alleles) have also been identified. These alleles alter P-protein levels by controlling the amount of OCA2 RNA that is generated. The allele that results in high levels of P-protein is linked to brown eyes. Another allele, associated with blue eye color, dramatically reduces the P-protein concentration.

On the surface, this sounds like the dominant/recessive eye color model that has been taught in biology classes for decades. However, while about three-fourths of eye color variation can be explained by genetic changes in and around this gene, OCA2 is not the only influence on color. A recent study that compared eye color to OCA2 status showed that 62 percent of individuals with two copies of the blue-eyed OCA2 allele, as well as 7.5 percent of the individuals who had the brown-eyed OCA2 alleles, had blue eyes. A number of other genes (such as TYRP1, ASIP and ALC42A5) also function in the melanin pathway and shift the total amount of melanin present in the iris. The combined efforts of these genes may boost melanin levels to produce hazel or brown eyes, or reduce total melanin resulting in blue eyes. This explains how two parents with blue eyes can have green- or brown-eyed children (an impossible situation under the Davenport single gene model) – the combination of color alleles received by the child resulted in a greater amount of melanin than either parent individually possessed.

As a side note, while there is a wide variability in eye color, colors other than brown only exist among individuals of European descent. African and Asian populations are typically brown-eyed. In 2008 a team of researchers studying the OCA2 gene published results demonstrating that the allele associated with blue eyes occurred only within the last 6,000 – 10,000 years within the European population.

Pigmentation research at HudsonAlpha Dr. Greg Barsh, a physician-scientist who has recently joined the HudsonAlpha faculty, and his lab study key aspects of cell signaling and natural variation as a means to better understand, diagnose and treat human diseases. In particular, his work has focused on pigmentation disorders. He has explored mutations that affect easily observable traits—such as variation in eye, hair or skin colors—as a signpost for more complex processes such as diabetes, obesity, neurodegeneration and melanoma, the most serious form of skin cancer.

Predicting eye color is more complicated than the Punnett square you probably learned about in high school biology as well. Scientists once believed eye color was determined by a single gene, but advances in genetic research have revealed that more than a dozen genes influence eye color.

This article discusses eye color genetics. It explains how genes trigger different combinations of pigments to determine what color your baby’s eyes will be.

The colored part of the eye is called the iris. What we see as eye color is really just a combination of pigments (colors) produced in a layer of the iris known as the stroma. There are three such pigments:

The combination of pigments, as well as how widely they're spread out and absorbed by the stroma, determine whether an eye looks brown, hazel, green, gray, blue, or a variation of those colors.

For example, brown eyes have a higher amount of melanin than green or hazel eyes. Blue eyes have very little pigment. They appear blue for the same reason the sky and water appear blue—by scattering light so that more blue light reflects back out.

When you don't have any melanin at all, you end up with the pale blue eyes of people with albinism.

A newborn's eyes typically are dark, and the color is often related to their skin tone. White babies tend to be born with blue or gray eyes. Black, Hispanic, and Asian babies commonly have brown or black eyes.

When a baby is born, pigment is not widely spread throughout the iris. During the first six months of life, more of the pigments are produced. By age 1, you usually have your permanent eye color.

Eye color is determined by multiple variations of genes that are in charge of the production and distribution of melanin, pheomelanin, and eumelanin.

The main genes influencing eye color are called OCA2 and HERC2. Both are located on human chromosome 15.

Each gene has two different versions (alleles). You inherit one from the mother and one from the father. If the two alleles of a specific gene are different (heterozygous), the trait that is dominant is expressed (shown). The trait that is hidden is called recessive.

If a trait is recessive, like blue eyes, it usually only appears when the alleles are the same (homozygous). Brown eye color is a dominant trait and blue eye color is a recessive trait. Green eye color is a mix of both. Green is recessive to brown but dominant to blue.

Other genes that help determine eye, skin, and hair color include the genes ASIP, IRF4, SLC24A4, SLC24A5, SLC45A2, TPCN2, TYR, and TYRP1. These genes are thought to interact with OCA2 and HERC2 to decide eye color.

Without knowing exactly which genes a baby will have, it's impossible to predict with total certainty what color their eyes will be. But there are ways to make fairly accurate predictions.

One of these is by using a simple grid chart called the Punnett square. You enter the genetic traits of one parent in the top rows of the grid. The other parent's genetic traits are entered in the far-left columns. Plotting the contribution each parent makes provides a better-than-average probability of what their child's eye color will be.

Determining each parent’s alleles can get a little complicated depending on the eye color. As a dominant trait, brown eyes can come from six different genetic combinations. They can also hide recessive (hidden) traits of green or blue eye color. To find any recessive traits, it's helpful to know the grandparents' eye colors.

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.

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.

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.

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).

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.

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.

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.