Re: What makes us have diffrent eye color?

Hi,

Our eye colour is basically determined by the combination of colour 
pigments on our iris (i.e. the ring of muscle that control the diameter of 
our pupil, which is controlled by reflex involving the mid-brain). In 
essence, there is a pigment called melanin that gives rise to colour in 
the hair, the skin and the iris and the choroids layer of the eyes. 
Melanin is contained within special cells called melanophores, and is 
produced by the metabolism of the amino acid tyrosine. There are a number 
of different type of melanins, and different combination of the different 
melanins give rise to the difference in our eye colours. 

Take hair colouration as an example, hair pigments, like all other visible 
pigment in mammals, result from synthesis and distribution of melanin, in 
this case, in the hair bulbs. In the case of hair colouration there are 
two basic types of melanins: eumelanins, which are brown or black; and 
phaseomelanins, which are red or yellow. Black human hair contains 
approximately 99% eumelanin and 1% phaseomelanin, brown and blond hair 
contain 95% eumelanin and 5% phaseomelanin; and red hair contains 67% 
eumelanin and 33% phaseomelanin.

Please note that the type of melanins produced by an individual is 
genetically determined, this is why people coming from different parts of 
the world have different eye colour, and the colouration of the eye is 
highly correlative to the colouration of other pigmented tissue in the 
body, namely the skin and the hair. You can observe an interesting 
phenomenon here: if a child whose dad has dark coloured eye and whose mom 
has light coloured eye (or vice versa of course!!) the child would 
normally have darker eye colouration. This is because the inheritance of 
dark pigment simply “mask” the lighter colours.

Isn’t this just amazing that across the spectrum of people in this world 
we have so many different variation of eye colours, skin colours and hair 
colour?! I hope you will agree that the world is just so beautifully 
colourful!

Joshua Chai
Medical student
University of Cambridge