Writing about the twins of Cândido Godói last month got me thinking about another extreme example of genetic bottlenecking and the resulting founder effect on future generations, namely the high percentage of persons suffering from complete colour blindness (complete achromatopsia) on the Micronesian atoll of Pingelap in Pohnpei state. Sufferers of ‘maskun’, as it is known in Pingelapese, know the world as nothing but black, white, and shades of grey. This makes visual acuity in daylight quite poor (sufferers have less than 20/200 vision in daylight). As a result, persons afflicted with achromatopsia work and play better at night, since they can only perceive tones rather than colours.
So how just how prevalent is this extremely rare affliction on Pingelap? 10% of the population is directly affected, with another 30% unaffected but carrying the gene. This all began in 1775 with the arrival of Typhoon Lengkieki, which killed 90% of Pingelap’s population. One of the survivors was Nahnmwarki Mwanenised, the ruler of the island and evidently an unaffected carrier of achromatopsia, and all achromats on the island today can trace their ancestry to him. As long as two people descended from Mwanenised never interbred, no achromatic children would have been born. With such a small, isolated population, however, some level of inbreeding was inevitable if the island was to be repopulated. It was in the fourth generation after the typhoon (that is to say, two second-cousins breeding) that the first afflicted persons were born, and from that point onward, the condition spread to its current level.
The British-American neurologist/author Oliver Sacks, most famous for his book-turned-movie Awakenings, was so taken by the story that he produced both a book and documentary entitled The Island of the Colorblind. Part 1 of the documentary is embedded below; the entire film can be found on this YouTube playlist.
Beardsley, F.R. (1998). Pingelap. Micronesian Diary, 12 October 1998. Available at http://www.intangible.org/Features/micronesia/text/Pon3.html. Accessed 15 August 2010.
de Castro, E. (1998). Incomplete and Complete Achromatopsia. 1998 Second Web Reports. Bryn Mawr, PA: Bryn Mawr College. Available at http://serendip.brynmawr.edu/exchange/node/1816. Accessed 15 August 2010.
Hussels et al. (1972). Pingelap and Mokil Atolls: Achromatopsia. American Journal of Human Genetics 24(3): 304-309. Available at http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1762283/?tool=pmcentrez. Accessed 15 August 2010.
Morton, N.E. et al. (1972). Pingelap and Mokil Atolls: historical genetics. American Journal of Human Genetics 24(3): 277-289. Available at http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1762283/?tool=pmcentrez. Accessed 15 August 2010.
Sands, O. (1998). The Island of the Colorblind. New York: Vintage Books.
von der Fecht, H. (2009). The People of Pingelap. Foto8, 15 February 2009. Available at http://www.foto8.com/new/online/photo-stories/793-the-people-of-pingelap. Accessed 15 August 2010.