“Hey, check out that guy. He has such mesmerising grey eyes!” – While some people have preferences over specific eye colours, some people don’t. Speaking of which, why do some people have green eyes while others have brown or blue eyes? Isn’t it fascinating how the human eye colours can come in so many different colours and iris patterns?
Nowadays, with contact lenses that come in a wide range of colours easily available, people can easily ‘transformed into” a red-eye vampire one day and a yellow-eye werewolf the day after. However, many of those who have preferences over specific eye colours can’t help hoping that one day, they can genetically engineer themselves so that they can have the desired eye colour without having to wear contact lenses.
Originally thought to be only controlled by a gene where brown is dominant over blue, the human eye colours are actually polygenic.
The secret behind all those different eye colours lie in our genes. Originally thought to be only controlled by a gene where brown is dominant over blue, the human eye colours are actually polygenic and are the result of pigmentation in the iris surrounding the pupil. Iris has an important role in controlling amount of light entering our eyes and its colour is determined by genes involved in the production, transport, or storage of melanin pigment.
Eye colour is directly related to the amount and quality of melanin in the front layers of the iris. The amount of melanin pigment in the iris, together with white collagen fibres, gives rise to the eye colours that we are seeing.
Two genes on the human chromosome 15 have a major role in determining our eye colour – OCA2 and HERC2. OCA2 gene codes for P protein, a protein involved in the maturation of melanosomes, which are important for melanin pigment production and storage. HERC2 gene on the other hand, plays important role in regulating the expression of OCA2 gene.
The P protein encoded by OCA2 gene determines the amount and quality of melanin pigment present in the iris. A reduction in functional P protein production will lead to less melanin pigment in the iris.
Similarly, for HERC2 gene, reduced function of HERC2 protein will lead to reduced OCA2 gene expression and thus less melanin pigment in the iris. When light travels through a relatively melanin-free iris, collagen fibres within the iris scatter the short blue light to the surface creating the blue appearance of the iris, and hence giving rise to blue eyes instead of brown.
Apart from HERC2 and OCA2 genes, several other genes - ASIP, IRF4, SLC24A4, SLC24A5, SLC45A2, TPCN2, TYR, and TYRP1 – have also been known to have some roles in determining eye colour. It is known that these genes, together with OCA2 and HERC2 genes, give rise to various shades of eye colours in different people.
Over fifty percent of the world's population has brown eyes, with nearly all individuals from Africa and Asia sharing brown eye colour
Several disorders, such as ocular albinism and oculocutaneous albinism, have been described to affect eye colours of the patients. These disorders affect the pigmentation of the iris, or even the skin and hair for oculocutaneous albinism patients.
As a result of reduced pigmentation, patients with these disorders will have light-coloured eyes. Sadly, they will also suffer from significant vision problems such as reduced sharpness; rapid, involuntary eye movements (nystagmus); and increased sensitivity to light (photophobia).
While some people find certain eye colour to be more attractive, it is important to bear in mind that good personality traits are far more important than how stunning someone’s eye colour is. Plus, the colour that you might be seeing might be the colour of the contact lenses? Oh, and by the way, my eyes are red not because I am a vampire but because of lack of sleep.