Genetics of Eye Colour: What does your eye colour says?
Hopefully many of you have come across the people with different eye colour. Dark brown is the most common one, while we also get to see blue, green/hazel and light brown too. Lighter eye colours like blue and green are found exclusively among the people of European ancestry. A person’s eye colour is due to the pigmentation of iris, which surrounds the black dot in the centre of the eye.
In humans, the pigmentation of iris varies from light blue to dark brown. It depends on the concentration of melanin in the iris pigment epithelium, within iris stroma and the cellular density of the stroma. Melanin pigments are the derivatives of the amino acid tyrosine. Eumelanin, Pheomelanin and Neuromelanin are the three naturally occurring melanin. Eumelanin and Pheomelanin are responsible for eye, hair and skin colour.
Neuromelanin colours certain distinctive regions of the brain. People with black or brown eyes have more melanin to block the sun’s rays, while those with blue, green/hazel eyes have less melanin. Thus, light coloured eyes have little protection from the sun and may experience discomfort, irritation, burning and tissue damage if the eyes are not protected by sunglasses when exposed to bright light. Snow blindness is an example of this kind of damage.
Let’s understand Genetics of Eye Color
The genetics of eye is complicated as it depends on multiple genes. Eye colour is determined by variation in a person’s genes. The actual number of genes that contribute to eye colour is not yet known, but so far it is detected that fifteen genes have been associated with eye colour inheritance. The two genes OCA2 and HERC2 play a major role in eye colour which is located in a particular region of chromosome-15.
A protein called P-protein is produced by OCA2 gene which is involved in the maturation of Melanosomes. Melanosomes are the cellular structures that produce and store melanin. Less P-protein means less melanin in the iris, leading to blue eyes instead of brown. Intron 86 is a region on HERC2 gene that has a segment of DNA which controls the expression of OCA2 gene. This can lead to lesser melanin in the iris and lighter coloured eyes.
Several other genes also play a smaller role in determining eye colour. Some of these also contribute to skin and hair colouring. Genes with reported roles in eye colour include ASIP, IRF4, SLC24A4, SLC24A5, SLC45A2, TCPN2, TYR and TYRP1. The effects of these genes likely combine with those of OCA2 and HERC2 to produce a continuum of eye colour in different people.
The inheritance of eye colour is more complex than suspected as it involves multiple genes. A child’s eye colour can often be predicted by the eye colours of his or her parents and other relatives, and genetic variations sometimes produce unexpected results.
There are several disorders that are associated to eye colour.
- Ocular albinism leads to severely reduced pigmentation of iris, which causes very light coloured eyes and severe vision problems.
- Oculocutaneous albinism also effects the pigmentation of eyes along with the pigmentation of skin and hair. Affected person tend to have light coloured eye, fair skin and white or light coloured hair. Both Ocular albinism and Oculocutaneous albinism are the result of gene mutation involved in the production and storage of melanin.
- Heterochromia is a condition of different coloured eyes in the same individual. It may be due to genetic changes or improper eye development.
Thus, the colour of your eye is not about your beauty, but about your genes and health. The colour of your eye, hair and skin says a lot about your genetic status.