By Paul Rincon
Science reporter, BBC News
Scientists have made a breakthrough in their understanding of the genetics behind human eye colour.
Eye colour has always intrigued geneticists
They found that just a few "letters" out of the six billion that make up the genetic code are responsible for most of the variation in human eye colour.
The research, by a team of scientists from Queensland, Australia, will appear in a forthcoming issue of the American Journal of Human Genetics.
The findings are based on a genetic study of nearly 4,000 individuals.
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.
Brown and blue
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.
THE DNA MOLECULE
The double-stranded DNA molecule is held together by four chemical components called bases
Adenine (A) bonds with thymine (T); cytosine(C) bonds with guanine (G)
Groupings of these "letters" form the "code of life"; there are about 2.9 billion base-pairs in the human genome wound into 24 distinct bundles, or chromosomes
Written in the DNA are about 20-25,000 genes which human cells use as starting templates to make proteins; these sophisticated molecules build and maintain our bodies
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.
Altogether, the single letter changes identified in the study accounted for 74% of total variation in eye colour, the researchers said.
The study was a collaboration between researchers at the Queensland Institute of Medical Research and the University of Queensland, both in Brisbane.