Eye Colour

More than meets the eye...

7 minute read

We often think of eye color in simple rules. Two parents with blue eyes will have children with blue eyes. With parents with brown eyes, it depends on what their parents had. For many people, the concept of a ‘recessive’ gene is taught this way. That is, you need two copies of the ‘blue’ gene to have blue eyes - if you get one ‘brown’ gene or ‘hazel’ gene then you will have darker eyes. It is a slight oversimplification, but in general brown is stronger than hazel which is stronger than blue.

Of course, these rules miss a tremendous amount of the complexity in eye color. There is tremendous diversity in eye color, including brown, hazel, blue, grey, and even violet. Two parents with blue eyes can have children with a darker shade. People can even have differences to their left and right eye, ranging from slight changes in hue to dramatic contrast.

There are more than 16 different genes that have been shown to influence eye color. However, two of them rise above the rest. A gene called OCA2 codes for melanin, a protein that is present in the eye which blocks light and gives it a dark pigment . A small sequence of DNA several hundred thousand bases away from OCA2 acts as an on/off switch for the gene. Small changes to this switch can have a dramatic effect on the amount of pigment produced, and therefore eye color. For instance, people with the letter T on position 28285036 of chromosome 15 produce less pigment and are 5-10 times more likely to have blue eyes than brown eyes. People with a G in this position produce more pigment and are more likely to have brown eyes. You can check your DNA at this position, called rs1667394, and see how it compares to your eye color.

rs1667394

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Besides the changes to the on/off switch of OCA2, there can be changes to the sequence of OCA2 gene itself, which affects the structure of the melanin pigment produced. For instance, people with an A at position 27985172 on chromosome 15 are more likely to have green, hazel, or black eyes while people with a G at the same position are more likely to have blue or grey eyes. You can check your DNA at this position, called rs1800407, and see how it compares to your eye color.

rs1800407

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The OCA2 gene was actualy originally discovered due to it’s role in , an inherited condition in which the amount of pigment present in the skin, hair, and eyes is reduced. In fact, there are 7 different ‘OCA’ genes (named OCA1, OCA2, …, OCA7) which are similar in their sequence, but produce slightly different proteins. Variation in many of these genes can cause alibinism as well as more common variation in eye/hair color.

The eye is just the beginning

Because many of the pigments present in the eyes are also present in hair and skin, genetic variation that has an impact on eye color often has an effect on hair and skin as well. For example, people with the letter T in rs1667394 (described above, the site with the strongest known effect on eye color) are also 5x more likely to be have blond hair than brown hair.

The second site mentioned above, rs1800407, has also been identified as a risk factor for melanoma, a form of skin cancer.

As genetics research continues to involve larger and larger numbers of people, understanding of the genetic basis of traits continues to improve, as does our understanding of the connections between traits. It is also really important to note that most of these relationships have been studied much more heavily in people of European ethnicity than other parts of the world. Different genetic variants may have different effects in different people, and it is very important for scientists to continue to expand their studies to include as much of human diversity as possible.

Image credit: cloudypixel - Unsplash

References

[1]Association of poly-morphic sites in the OCA2gene with eye colour using the treescanning method.

[2]Genetic determinants of hair, eye and skin pigmentation in Europeans.

Glossary

[Albinism]

Albinism is a congenital disorder characterized in humans by the complete or partial absence of pigment in the skin, hair and eyes.

[SNP]

SNP stands for 'single nucleotide polymorphism' and refers to regions of DNA that vary between individuals.

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