Eyes come in various shades and hues such as brown, green, blue, hazel, and gray. These are known as phenotypic traits. Rare genetic mutations can result in more unusual colours like black, red, and violet. And heterochromia is a condition where the eyes are a different colour, so a person may have one brown eye and one blue eye.
Eye Colour Pigment
Eye colours are created by the pigment producing cells known as melanocytes.
Brown eyes – the most common eye colour in humans, resulting from a considerable amount of melanin in the iris.
Blue eyes – by comparison to brown eyes there are lower levels of melanin present in the iris.
Hazel eyes – contains an above average amount of melanin in the iris.
Eye Colour Genetics
The classroom version of eye colour genetics generally holds true, that brown eye genes are dominant over green eye genes, and both brown and green eye genes are dominant over blue. However, the biological reality is not as simple as that because eye colour is a polygenetic genetic, it involves many different genes, some of which remain unknown to science. And each gene comes in two versions which leads to a considerable amount of variation. So for example, it is possible for two blue-eyed parents to have a child with brown eyes. Although this would seem to be contra to what we were taught at school.
Eye Colour Inheritance
Recombination – during the shuffling of genes when the gametes are formed a parent's eye colour genes could be slightly modified.
Polygenetic trait – suppose that there are five genes that lead to brown eye colour – A, B, C, D, E and blue eyes result from mutations in both copies of any one of these genes. Now consider two blue-eyed parents. Dad's blue eyes are because of mutations in both copies of gene C. And mum's blue eyes are due to mutations in both copies of gene E. But if mum gives a child a brown version of C and dad gives a brown copy of E then the child will have brown eyes. Eye colour inheritance does not necessarily follow the inheritance pattern you might expect to see with a single trait.
In 2006 a team of researchers from Queensland University in Australia published a paper demonstrating that the genetics of eye colour variation is down to just a few subtle changes in the genetic code. The scientific team studied nearly 4,000 individuals and concluded that the differences in eye colour are down to single nucleotide polymorphisms (SNPs – pronounced "snips") – a snip is a change in just one letter of the genetic code.
All the snips relating to eye colour were found near a gene called OCA2 – this gene codes for a protein that helps give hair, skin, and eyes their colour. The scientists don't believe that the snips are causing functional changes in eye colour, more they are linked to something else that is. It could be that different types of snips regulate the amount of pigment that is produced by OCA2. And the amount of pigment produced determines eye colour.
It is a complicated and fascinating topic and we are beginning to see more clearly the fundamental genetic basis of eye colour.
"No Single Gene For Eye Color, Researchers Prove." ScienceDaily 25 Feb. 2007. Web. 5 Apr. 2011