Complete achromatopsia is normally a very rare condition, and its prevalence on the island has been traced back to a population bottleneck in 1775 after a catastrophic typhoon swept through the island, leaving only about 20 survivors.

There are now 250 people on the island, and…

the disorder is now prevalent in almost 10% of the population, with a further 30% being unaffected carriers.

So ~25 people have it, and another ~75 are carriers.