Sometime around 1820, a French orphan named Martin Fugate, carrier of an incredibly rare recessive gene for a disease known as hereditary methemoglobinemia, settled on the banks of Troublesome Creek in Eastern Kentucky and married Elizabeth Smith, carrier of the same incredibly rare recessive gene.
It was a remarkable coincidence with a bizarre result: four of the seven Fugate children were born with bright blue skin that lasted their entire lives. (According to legend, and the image above, Martin Fugate himself had blue skin). The reason for this strange skin disorder was only discovered a century later when it was realized that due to an enzyme deficiency, the Fugates' blood had a diminished oxygen-carrying capacity.
Over the years, the Fugates interbred repeatedly. Blue people proliferated. Six generations later, according to a Science article published in 1982, there were still Blue Fugates roaming the hills of eastern Kentucky.