08 July 2007

The Founder Effect

One of the basic principles of population genetics is the founder effect. Basically, if an isolated population has its roots in a small number of founding members, later generations will look a lot like the founding members genetically. So, if the people who broke away from another group to found the new population were atypical of the people they left in some random way, those anomolies will persist in the founding group's descendants.

The classic isolated population is the population of an island. Also, small populations are also more prone to random variations than large populations. So, we expect small isolated groups of people, like the 17,000 people of the island nations of Palau to have quirky population genetics.

There is strong evidence that this is the case when it comes to schitzophrenia incidence in Palau.

Genetic transmission plays a major role in the pathogenesis of schizophrenia. Family, twin, and adoption studies have consistently shown that risks in relatives are many times greater than the general population risk of 1%. McGue, Gottesman, and Rao (1983; Am J Hum Genet 35:1161-1178) calculated risk estimates of 12.8% for offspring and 3.5% for nieces/nephews of schizophrenia patients based on a large data set of Western European families. The present study evaluated corresponding risk levels in Palau, an isolated population in Micronesia where the prevalence of narrowly (broadly) defined schizophrenia is 1.99% (2.67%) and cases cluster in extended pedigrees, 20 of which contain 80% of affected individuals. . . . Risks to 1st- and 2nd-degree offspring were approximately double the rates found in the smaller Western European families: 23.4% in the offspring of an affected parent, 6.4% in offspring with one affected aunt/uncle, and 15.0% in offspring with two or more affected aunts/uncles. Recurrence rates in offspring of an affected parent were 1.6 times higher in males (27.9%) than in females (17.7%). The high risk levels we found in Palauan offspring reflect the elevated population prevalence, strong familial aggregation, and multi-lineal transmission pattern of schizophrenia in Palau.

If the effective founding group of Palau was just 50-100 people, just one or two founders with schitzophrenia could account for this high incidence of schitzophrenia now. Similar results have been seen in the case of achromatopsia incidence on the Micronesian atoll of Pingelap.

Hat Tip to Science News.

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