The genetics behind eye color have been discovered:
Differences in eye colour are largely down to “single nucleotide polymorphisms” (SNPs – pronounced “snips”); variations in the sequence of letters that make up a single strand of human DNA.
SNPs represent a change of just one letter in the genetic sequence. These changes, or mutations, in our DNA can have important consequences for how the gene gets physically expressed.
All the SNPs are located near a gene called OCA2. This gene produces a protein that helps give hair, skin and eyes their colour. And mutations in OCA2 cause the most common type of albinism.
The study, which focused on twins, their siblings and parents, shows – conclusively – that there is no “gene” for eye colour.
Everyone has two copies of a SNP. So there are several possible combinations, some of which are more heavily associated with, for example, blue eyes, than with brown eyes.
In short, these combinations strongly influence the colour of a person’s eyes, but they are not the final word.
Dr Richard Sturm and his colleagues found three SNPs near the start of the OCA2 gene that were linked to blue eye colour.
“The SNPs we’ve identified in themselves are not functionally causing the eye colour change, but they are linked very, very closely to something that is,” Dr Sturm, from the University of Queensland, told BBC News.
“When OCA2 is knocked out, there is a loss of pigmentation. The position of these SNPs right at the start of the gene means it is possible we’re looking at a change in the regulation of the gene in people with blue eye colour.”
So these SNPs, at the start of OCA2, probably regulate how much of the pigmentation protein is produced by the gene. People with brown eyes might have a lot of this protein, while people with blue eyes have less.
However, the single letter changes involved in green eyes may actually produce functional changes in the pigmentation protein.
The researchers found SNPs at another position in the OCA2 region – linked to green eyes – that resulted in changes to amino acids (the building blocks of a protein).
“To use an analogy, one of the changes is like switching the light on and off, while the other is like changing the light bulb from brown to green,” said Dr Sturm.
I think this is pretty cool.