The Genetics of Human Eye Colour
Eye colour is an inherited trait influenced by more than one gene in the human population. Eyes can range in colour from the most common, brown, to the least common, green.
Although the actual number of genes that contribute to eye colour is currently unknown, there have been several studies, one of which (Rotterdam, 2009) found that it was possible to predict the colour of eyes with more than 90% accuracy for blue and brown eyes using just six SNPs (single-nucleotide polymorphisms, pronounced snips: a DNA sequence variation that occurs when a single nucleotide in the genome differs between members of a biological species, or paired chromosomes of an individual. They account for many of the variations in the way humans look). We do know, however, that eye colour is largely regulated by a gene called OCA2, which, when in its variant form, causes the pink eye colour and hypopigmentation seen in human albanism. Different SNPs within OCA2 are strongly associated with blue and green eyes (eyes with less pigmentation), as well as freckling, mole count, and skin tone.
Interestingly, it is believed that blue eyes with a brown spot and gray eyes are caused by entirely different sections of the genome.