What determines your eye colour?

What determines your eye colour?

Eye colour – where does it come from?

Eye colour (or colour of the iris) is not as predictable as one might think, even though it is inherited from the genes of biological parents. Eye colour has a lot to do with genetic material shared with parents, but is not necessarily the predictable blend one might expect. It’s not like mixing paint colours. The inheritance pattern is a little more complicated than that when it comes to colour (pigment).

The inheritance process of genetic traits, in general, is a fairly complex one. Two brown-eyed parents may produce blue-eyed offspring, but a pair of blue-eyed parents are less likely to have darker-eyed babies. Simply put, blue-eyed genetic traits are less dominant than darker ones, and were previously termed ‘recessive’. Less dominant blue-eyed traits can be passed along by brown-eyed individuals, however, until such time as the lighter colour matches up, many generations later. A parental mix of brown and blue also doesn’t necessarily produce a brown-eyed child (with brown being the more dominant trait).

New parents can get quite excited in the lead up to the day a baby is born. What will he or she look like? Will the baby have her eyes and his nose? Will he or she have dark or light hair? What colour will the eyes be? Will the colour be solid or a combination?

The majority of babies born are expected to have blue eyes, with some darkening within their first 3 years of life. Darkening usually occurs if a brown pigment, known as melanin, develops with age (usually not present at birth). It is possible for a new-born to have a completely different eye colour to those of his or her parents.

It is also possible for babies to be born with irises that are mismatched. This may be as a result of an impaired transport of developmental pigment, a possible trauma having occurred in the womb during birth or even a benign genetic condition that has developed during pregnancy.

Mismatched irises could also be as a result of a freckle of the iris (diffuse nevus), the development of inflammation, or the occurrence of a disrupted nerve pathway on one side of the brain affecting the eye such as Horner’s syndrome.

So, what are the genetic traits that determine eye colour and just how complex a process is it?

It’s all in the genes

Iris of the human eye.A child is formed through a complex mix of genetic material (chromosomes) from either conceiving parent. This then mixes and matches in different ways to produce a unique little human being.

Pigmentation in the iris, the coloured circle that surrounds the pupil, is what determines the colour of the eye. This pigment effectively originates from at least three different genes, which then determine the most dominant colours – blue, brown and green. The precise nature of how these genes determine colour is still being researched. Not all genetic material is yet fully understood in relation to the formation of pigment.

Iris-holding pigment also helps to control just how much light will be allowed to enter the eye (how light is scattered throughout once passed through). Melanin works in conjunction with white collagen fibres, which help to produce varying shades of green, hazel and grey. In the case of a relatively melanin-free iris (light coloured eyes), the collagen fibres scatter any blue light coming through to the surface, thus creating a blue looking iris (blue eyes).

Iris colour patterns exist in gradient shades of blue to dark brown, and every colour in between. Thus, pigment ranges from a very light blue colour, to darker shades of blue, green and hazel to an intensely dark brown / black.

Being the dominant colour, brown is the more common eye colour across the world. Blue and green eyes are predominantly found amongst those with a European ancestry.

Researchers from all over the world are working on finding accurate ways to predict eye colour, including variable eye colours, through developing sophisticated DNA analysis. Dutch researchers have been working to achieve at least 90% accuracy using these analysis processes. Through analysis, researchers hope to gain an accurate understanding as to just how genetics directly determine a person’s eye colour.

Once this research accomplishes its goals, eye colour prediction may have other uses other than just for interest’s sake, such as in forensic investigations. If DNA recovered from a crime scene can be analysed to the point of being able to accurately determine details of a suspect’s appearance, these clues can have a positive impact for resolving wrongdoing in society.

How are genes thought to determine eye colour?

Set of open, different coloured eyes.It all starts with a collection of chromosomes. The genetic material that ‘carries the code’ for human form, and effectively develops and maintains the body (throughout a person’s lifetime), is contained in what is known as chromosomes. All inherited traits come from these genes which form every cell of the human body. Within each cell is a nucleus where genetic material is stored from the time of conception to the end of a human life. Genes are thus exclusively responsible for how the human body looks and functions.

Genes effectively function as instructions for the production of proteins. Genes are structured in two primary parts – to carry the code for instructions to produce a protein, and carry non-coding instructions which prompt where and when a protein is needed to be made, as well as how much is required.

Once an egg and sperm become fertilised, forming an embryo, 23 pairs (or copies) of chromosomes (46 in total) begin the process of creating a human being. One half of the chromosome pairs comes from a woman’s egg cell (ovum) and the other from those of a man’s sperm. The correct number effectively creates the most ideal form. Any abnormal division can result in genetic defects.

The production, transport and storage of the pigment, melanin (two black and yellow pigments) is dependent on various factors concerning individual genetic material. The amount and quality of this pigment in the 2 outer layers of the iris has a direct involvement in the development of eye colour.

This means that a higher amount of melanin in these layers in the iris (more black and yellow pigments) contributes to darker colour eyes. Less melanin results in lighter colour eyes.

Research has determined that a region on chromosome 15 has direct involvement in the development of eye colour. Two genes which are located very close together have also been determined as having direct involvement because they form part of chromosome 15. Thus, the two best understood genes are OCA2 and HERC2.

The OCA2 gene was formerly known as the P gene (or P protein). This gene is responsible for the production of P protein, which is responsible for the production of melanin through a maturation process of melanosomes. Melanin not only is linked to eye colour, but has a direct role in the shade of skin and hair too. Polymorphisms in this gene (i.e. genetic variations) can result in a lower amount of P protein that is functional, which results in less melanin in the outer layers of the iris. Thus brown-eyed people have a higher amount of polymorphism in their genes, producing higher quantities of melanin in the iris. The OCA2 gene is thus a key factor in what determines the colour of a person’s eyes, and the shade.

An impairment that occurs with this gene can also lead to the kind of results seen with certain types of albinism. When pigmentation of the iris is severely low, ocular albinism can occur, resulting in very light-coloured eyes with accompanying vision problems. Eyes that are very light in colour and are accompanied by fair skin and light-coloured hair (including white) can also occur, and is known as oculocutaneous albinism.

The HERC2 gene is also known as intron 86. This contains a DNA segment which influences expression (or controls activity) of the OCA2 gene. This ‘control’ function effectively turns the expression of the OCA2 ‘on and off’ as needed.

Lighter coloured eyes may have more to do with the non-coding region of the HERC2 gene. The theory is that a change in colour must be where the protein is made (i.e. not a change in the protein itself). It was previously thought that that non-coding region of the OCA2 gene was responsible, but subsequent research has pointed to the HERC2 gene instead.

Research has determined that at least one polymorphism contained in the HERC2 gene can reduce OCS2 expression, which in turn lowers melanin production, thus resulting in lighter colour eyes. Effectively a ‘non-working’ OCA2 gene is one of the determining factors for lighter coloured eyes (and not just blue shades).

The HERC2 non-coding region effectively functions like a switch. What research has been able to pinpoint relates to what is known as transcription factors (TFs). These are special proteins which are, theoretically, capable of recognising portions of DNA in order to bind them. When turned on in nearby genes (such as the HERC2 gene), a TF results in binding where it hadn’t occurred before, resulting in the switching off of the OCA2 gene.

Most human cells contain the same DNA but not all function alike, and we can see this in the way the body works. A skin cell does not function in the same way those in the brain are required to. All of the body’s different cells have different transcription factors which are switched on and off.

A non-coding HERC2 gene region that contains a blue-eyed version is switched off and results in blue colour eyes. Non-coding portions containing brown eyed versions result in enough P protein to produce darker eyes. If a child receives a brown version of the OCA2 gene (i.e. where the OCA2 gene is turned on) from one parent and a blue version (i.e. where the OCA2 gene is turned off) from the other, the more dominant of the two is likely to result in brown-eyes (overriding the lighter colour because it produces more pigment).

This effectively explains how genes influence blue or darker eye colours. Research has not yet been able to pinpoint the genes directly responsible for green eyes. This is possibly because there may be more than one gene involved, not just versions of the OAC2 and HERC2 genes.

Genes hold the key, and contain the code of instructions for the body. Genes effectively are the manuscript by which the body ‘is told to function’. Genes hold the instructions for proteins, and it’s these proteins which do the actual work in the body’s cells.

Different colour eyes are not based on a set of ‘different colour based genes’. Difference in colour is based on distinct versions of the same protein / gene. Every person has a different version of the OCA2 gene. The variations of the P-protein produced by the gene influences whether a person has less of it, very little at all, a weak form of it, or more.

A single eye colour is obtained in multiple ways (polygenic traits / multiple genes). How different genes and their unique versions work together has a multi-layered effect on how a final eye colour is produced. Every layer has a function, much like a factory full of workers doing their bit to produce a final product. The same applies to the body.

Close-up of a blue-eyed man.

You may have heard the theory that all blue-eyed people in the world today are descendants of one ancestor who existed some 6 000 years ago.

Scientists are certainly curious about this and are researching genetic material to gain a better understanding. The theory, has focussed on trying to establish just where blue eyes actually come from in the world. Even when samples of participants were looked at from different parts of the world (initially just Europe), the same DNA difference in HERC2 genes were noted. Many of the participants, however, were not related. Researchers are looking into potentially more than one version of the OCA2 gene which could be a reason so many unrelated individuals develop blue eyes – such as in the case of red hair (there are at least 4 versions of the MC1R gene that causes red hair).

Some scientists have theorised that a blue-eyed mutation could have originated from the Black Sea region, with a population having migrated to Europe at least 6 000 to 10 000 years ago. Theories also include potential influences regarding vitamin D (specifically from sources such as sunlight) and lighter pigment skin. Perhaps ancestors also favoured certain aesthetic traits which also resulted in a growing number of blue-eyed people being reproduced (blue-eyed parents are more likely to produced blue-eyed children). One blue-eyed ancestor is theoretically (mathematically) possible, but is yet to actually be determined.

Theories also speculate that perhaps the human race all started with brown-eyed individuals, and due to genetic mutations, other colours developed through the ages. Perhaps this is why brown eyes are more common around the world?

Researchers have determined a variety of other genes linked to determining eye colour. Those that play a smaller role include:

  • ASIP
  • TYR
  • TYRP1
  • IRF4
  • SLC24A4
  • SLC24A5
  • SLC45A2

All of the above are believed to combine with the functions of OCA2 and HERC2 in order to produce an eye colour that is lifelasting. As many as 16 different genes, however, may have some influence on how colour is formed.

What does it mean when genes are described as dominant or recessive?

Researchers have determined that in order for certain human characteristics to appear, a person must have two alleles (variant forms of any given gene), which form part of genetic material. When a pair of alleles are the same, they are characterised as homozygous. When a pair is not similar at all, there are characterised as heterozygous.

One of the pair is dominant in nature (expressive) and the other recessive (non-expressive). Dominant features are typically the traits which appear, such as dark eyes. So why then is red hair considered recessive, and appears in certain people? If a pair of alleles are only recessive and one is not a dominant one, this can happen. This means that an unexpressed trait can appear.

The most common eye shades are brown and blue. It is already established that darker eye colour shades are more dominant over all colours. Green alleles are however, more dominant over those that are blue. Research believes that a blue-eyed person must have inherited blue alleles from both parents.

African American woman with dark-coloured eyes.

When eye colour changes …

Melanin production generally does not begin at birth. This is one reason why many babies born to Caucasian people have blue or blue-grey eyes. Once melanin production starts, a baby’s eyes may darken within the first few years, and the colour could change to become darker blue, green, hazel or shades of brown. Some babies are born with brown eyes and this colour doesn’t typically change over time. Usually by the age of 3, a permanent colour settles, which a person will likely have for the remainder of their life.

Eye colour, however, can appear to change for various reasons thereafter. Subtle changes can occur which alters colour a little. Sometimes, different lighting can have an effect on the colour of a person’s eyes. The reason a person’s eye colour may appear to change in different lighting is due to the pigment contained in each of the two layers in the iris. The front layer, in a person with blue or green eyes, for example, may have very little to no melanin at all. With diffraction of light and enough melanin in the other layer, a person’s eyes can appear as different colours in certain lighting conditions.

Other factors which influence eye colour during a person’s lifetime include:

  • Emotions: Intense emotions such as anger can alter the size of the pupil and thus influence colour as well. Eye colour can change slightly when the iris expands and contracts in order to control the pupil size. When lighting is dimmer, the pupil enlarges. In brighter lighting conditions, the pupil contracts and becomes smaller. During the process of expansion and contraction, the pigment in the iris spreads or compresses, which alters the colour of the eyes somewhat.
  • Age: For many, eye colour tends to lighten after a certain age. Generally, those with lighter colour eyes will notice that their colour lightens during their later years – for instance, colour can change from a hazel green to a shade of grey. Others may note that their dark eyes, darken instead. A small percentage of people (about 10% to 15%) can experience change in eye colour beyond their toddler years, and this is most commonly seen in people of Caucasian descent. Change is usually subtle though. Studies have noted that people with lighter colour eyes tend to experience more noticeable changes than those with darker coloured eyes.

When should you be concerned about eye colour changes?

Not every colour change is part of a normal functioning process. There are some instances where colour changes will need to be assessed / monitored by a medical doctor. Abnormal changes could be a result of one of the following:

  • Fuchs uveitis syndrome (Fuchs heterochromic uveitis / Fuchs heterochromic iridocyclitis): A condition which most commonly affects one eye, mild inflammation (usually chronic) alters the state of the middle portion of the eyes, including the iris. One of the resulting signs is a lightening of the iris (of the affected eye), which creates a difference in colour between a person’s eyes (heterochromia). This can increase a person’s risk of developing cataracts or glaucoma.
  • Pigmentary glaucoma / pigment dispersion syndrome: When the pigment in the iris becomes disrupted, loose granules accumulate in the front chamber (layer) of the eye. This can lead to one of two things – glaucoma (where vision loss occurs due to increased pressure in the eye) or dispersion (causing colour changes), which results in a person having two different colour eyes.
  • Horner syndrome: When an impairment of fibres in the third cranial nerve occurs, one of the signs of Horner syndrome is a lighter colour in the affected eye than the other. This is most commonly seen in children with the condition (most often before a child’s first birthday), but can also happen in adults (although rare). A key sign is a drooping eyelid. The pupil in the affected eye also contracts, becoming smaller than normal.
  • Medication usage: Prescribed medications for the treatment of glaucoma can also result in subtle changes in eye colour. Eye drops can help to reduce internal pressure in an affected eye and sometimes result in colour changes – lighter eyes may darken with an increased amount of pigment in the iris. In many instances, darkening as a result of medication use may be permanent.
    It is also possible for traumatic injury to the eyes or a tumour of the iris (benign or malignant) to cause changes in eyes colour. Any abnormal change in one or both eyes must be evaluated by a medical professional (eye specialist) as soon as possible, especially if there are symptoms of pain, redness or vision changes (blurry or limited ability).

Which eye colour occurs the most?

  • Brown eyes: More than 55% of the world’s population have brown eyes, predominantly located in Africa and Asia. For some, the dark brown colour can be so dark it appears black (contains an abundance of melanin).

Close-up of brown-coloured eyes.

  • Blue eyes: An estimated 8% of the world’s population have blue eyes, this is more frequently found in populations in northern Europe (near the Baltic Sea).

Close-up of blue-coloured eyes.

  • Hazel eyes: Although similar to brown eyes, this lighter shade has hints of green-yellow tints. The multi-coloured appearance is due to a higher concentration of pigment around the border of the iris which can appear both green and copper depending on the light. An estimated 5 – 8% of the world’s population are hazel-eyed.

Close-up of hazel-coloured eyes.

Are there eye colours that are rare?

  • Green eyes: Pure green eyes (not confused with hazel coloured eyes), are a little rarer, making up approximately 2% of the world’s population. Green eyes contain a mild amount of pigmentation in the iris with a golden tint. Green eyes are more common in northern and central Europe, and in some western Asian areas too.

Close-up of green-coloured eyes.

  • Amber eyes: A golden yellow or copper colour occurs due to higher quantities of the pigment lipochrome (yellow pigment) and very little melanin, and are considered very rare. Amber-coloured eyes are most often seen in Asian and South American areas of the world. A pure amber colour is more solid and uniform (hazel eyes, which amber colour can be confused with, are not), and appears to glow. The colour (nicknamed ‘wolf-eyes’) is more common in animal species than it is humans.

Close-up of amber-coloured eyes.

  • Silver (grey) eyes: A grey-silver colour is quite rare and occurs as a result of virtually no melanin in the iris. Silver eyes are considered to be one of the rarest colours around the world, but when they do occur, this is most often seen in eastern Europe areas.

Lady with silver-coloured eyes.

Pink or red colours can occur in those with albinism (those who usually have very light coloured eyes), due to a leaking of blood into the iris. Violet colours (a purplish blue) are also known to have occurred where a lack of pigment mixes with red light as it reflects off of red blood vessels in the eyes. Violet is more notably seen in those with albinism.

References

1.Genetics Home Reference. U.S. National Library of Medicine. May 2015. Is eye color determined by genetics: https://ghr.nlm.nih.gov/primer/traits/eyecolor [Accessed 31.07.2017]

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