Achromatopsia is a rare autosomal recessive form of complete color blindness that affects about 1 in 20,000 people in most populations. People with this disorder see only in black and white and have extreme sensitivity to light and poor visual acuity. On Pingelap Island, one of a cluster of coral atoll islands in the Federated States of Micronesia, approximately 10% of the 3000 indigenous Pingelapese inhabitants have achromatopsia. Achromatopsia was first recorded on Pingelap in the mid-1800s, about four generations after a typhoon devastated Pingelap and reduced the island population to about 20 people. All Pingelapese with achromatopsia trace their ancestry to one male who was one of the 20 typhoon survivors. Provide a genetic explanation for the origin of achromatopsia on Pingelap, and explain the most likely evolutionary model for the high frequency there of achromatopsia.

Key Concepts

Autosomal Recessive Inheritance

Autosomal recessive inheritance is a genetic pattern where two copies of a mutated gene, one from each parent, are required for an individual to express a trait or disorder. In the case of achromatopsia, individuals must inherit the recessive allele from both parents to exhibit the condition. This mode of inheritance can lead to a higher prevalence of the disorder in isolated populations, especially when a small gene pool exists.

Genetic Drift

Genetic drift is a mechanism of evolution that refers to random changes in allele frequencies within a population, particularly in small populations. In the context of Pingelap, the dramatic reduction in population size after the typhoon likely led to a bottleneck effect, where the genetic traits of the few survivors became more pronounced in subsequent generations, increasing the frequency of achromatopsia among the Pingelapese.

Founder Effect

The founder effect occurs when a small group of individuals establishes a new population, carrying only a fraction of the genetic diversity of the original population. In Pingelap, all individuals with achromatopsia can trace their ancestry to a single male survivor of the typhoon, which means that the genetic traits of this founder have a disproportionately large impact on the gene pool, leading to a higher prevalence of the disorder in the population.