Genetics of eye color unveiled

It's been discovered that only a few "letters" of the six billion that make up the human genetic code are responsible for most of the variation in the color of human eyes.

The investigation, conducted by a team of scientists from Queensland, Australia, will be published in an upcoming issue of the American Journal of Human Genetics.

The findings are based on a genetic study of nearly 4000 people.
The differences in the color of the eyes are largely a "single nucleotide polymorphisms (SNPs - pronounced" snips "); variations in the sequence of letters that form 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 may have important implications for the way the gene is expressed physically.

All SNPs are located near a gene called OCA2. This gene produces a protein that helps give hair, skin and the colour of his eyes. And OCA2 cause mutations in the most common type of albinism.

Brown and blue

The study, which focused on the twins, their brothers and their parents, it shows - conclusively - that there is a "gene" for the color of the eyes.
Everyone has two copies of a SNP. So there are a number of possible combinations, some of which are more strongly associated with, for example, blue eyes, with brown eyes.

In short, these combinations greatly influence the color of a person's eyes, but they are not the last word.

Dr. Richard Sturm and his colleagues found three HNS near the beginning of OCA2 gene that were associated with the color blue eyes.

"The SNPs that we have identified themselves are not functionally causing the change in eye color, but are linked very closely to something that is," Dr Sturm, of the University of Queensland, told BBC News.

"When OCA2 is eliminated, there is a loss of pigmentation. These SNP's position right at the beginning of the gene means that we are seeing a change in the regulation of the gene in people with color blue eyes."

Functional changes

So these SNPs, at the beginning of OCA2 probably regulate how much of the protein is produced by the pigmentation gene. People with brown eyes can have a lot of this protein, while people with blue eyes have less.

However, the only letter of the changes involved in the green eyes could produce functional changes in pigmentation of the protein.

The researchers found SNPs in another position in the region OCA2 - linked to the green eyes - which led to changes in the amino acids (the building blocks of protein).

"To use an analogy, one of the changes is like changing light and shuts down, while the other is how to change the bulb from brown to green," said Dr Sturm.

In total, the only letter of the changes identified in the study accounted for 74% of the total variation in the color of the eyes, researchers said.