|MadSci Network: Genetics|
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
Try the links in the MadSci Library for more information on Genetics.