The prevalence of Ellis-van Creveld syndrome in some Amish populations is an example of genetic drift and, specifically, of the founder effect. Ellis-van Creveld syndrome causes skeletal abnormalities and, often, heart defects.
Genetic drift is a change in gene frequency due to random or chance events. The founder effect is a specific type of genetic drift that affects small populations that remain small. The founder effect occurs in some populations of Amish in the United States, where it is responsible for higher than usual rates of genetic disorders, such as Ellis-van Creveld syndrome. Ellis-van Creveld syndrome is otherwise rare, occurring in roughly one out of every 60,000 to 200,000 births, but it is much more prevalent in the Old Order Amish of Lancaster County, Pa.
Nearly all Amish populations in North America descend from about 200 individuals, and Amish do not marry outside of their own culture and, sometimes, community. Because of this, a large number of individuals of Lancaster's Old Order Amish are either carriers or sufferers of Ellis-van Creveld syndrome. Ellis-van Creveld syndrome also occurs at a higher frequency in a small native population in Western Australia.
Ellis-van Creveld syndrome causes skeletal abnormalities, such as polydactyly, nail defects, dental defects and dwarfism. It also causes a serious heart defect in many of its sufferers.