Frank Sinatra was a mutant, according to scientific research into eye colour.
Originally everyone had brown eyes, and it wasn't until about 10,000 years ago that blue-eyed people started to appear.
Today it is estimated that 20 to 40 per cent of Europeans have blue eyes. So, where did the famous crooner known as 'Ol' Blue Eyes' and other blue-eyed people come from?
Short answer: genetic mutation. Eye colour is determined by the amount of pigment you have in your iris. If you have a lot of pigment you'll have brown eyes; less pigment and you'll have green or hazel eyes, and when there's little or no pigment, you'll have blue eyes.
So what causes your body to produce little or no pigment?
There are up to 16 genes involved in determining eye colour, but the two main genes are HERC2 and OCA2. HERC2 works as a switch to activate your OCA2 gene like a switch turns on a light.
Every one of us has two of each gene in their body, one of each from their mother and one of each from their father.
The OCA2 genes activate proteins which produce pigments. If the OCA2 genes are mutated, it can stop the chain of events that leads to pigments being produced.
A 2008 study at the University of Copenhagen looked at the DNA of blue-eyed people and found that 99.9 per cent of them had a mutation in their OCA2 genes. This led researchers to conclude that all blue-eyed may people may come from one single European ancestor.