Shown in this image is polydactyly—extra fingers or sometimes toes—one symptom of Ellis–van Creveld syndrome. The syndrome is commonly found among the Old Order Amish of Pennsylvania, a population that experiences the "founder effect." Genetically inherited diseases like Ellis–van Creveld are more concentrated among the Amish because they marry within their own community, which prevents new genetic variation from entering the population. Children are therefore more likely to inherit two copies of the particular recessive genes that lead to genetic disease.