By mutationWhat's behind the beauty of blue eyes?
An Australian team may have found the answer: just a few "letters" (nucleotide bases) out of the six billion that make up our genome are responsible for human eye color variation.
The genetic study of nearly 4,000 individuals showed that eye color differences are largely due to "single nucleotide polymorphisms" (SNPs): just the change of one letter in the whole genome. These mutations in our DNA can have important consequences for how the gene gets physically expressed.
All the SNPs related to variation in eye color are situated near a gene called OCA2 that encodes the pigment protein (melanin) that gives hair, skin and eyes color. The OCA 2 total switch off (in this case the organism cannot produce melanin at all) is the main cause of albinism. The study focused on twins, their siblings and parents and proved conclusively that there is no "gene" for eye color.
The team found three SNPs near the start of the OCA2 gene that were linked to blue eye color. "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 color", said Dr Richard Sturm, from the University of Queensland.
So these SNPs - at the start of OCA2 - probably just decrease the amount of melanin in eyes, without totally eliminating it. People with brown eyes might have a lot of melanin, while people with blue eyes have less. But the original variant of SNPs that produces brown eyes is dominant, so a blue-eyed person must have two mutated SNPs in order to possess blue eyes.
However, the single letter change involved in green eyes may actually produce functional changes in the melanin structure. The researchers found SNPs at another position in the OCA2 region that encodes green eyes and changed amino acids sequence in melanin structure. "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 color," the researchers said.