Color blindness
Rod monochromacy (achromatopsia): The retina has no cone cells. This makes it difficult to see even lights of a normal level; this means that people who suffer from it are almost blind. It occurs almost only on the island of Pingelap, a part of the Pohnpei state in the Federated States of Micronesia. There it is called maskun: about one person in 12 of the population has it. The island was hit by a storm in the 18th century. Very few men survived the storm. One of them carried a gene for rod monochromacy. Today, several hundred people live on the island, about 30% have this gene.