Founder effect

The founder effect refers to the situation that arises when an isolated population grows from a small number of individuals and therefore displays allelic frequencies that do not match what would be considered normal in the wider species, perhaps in part because of the effects of random chance due to genetic drift.

A famous example of the founder effect in humans occurs in the Amish community of Pennsylvania, where a member of one of the first Amish couples to settle in the area possessed a genetic disorder called Ellis-van Creveld syndrome, and to this day the disorder has been preserved in high percentages in the population.