The amish of lancaster, pa, have a higher frequency of an unusual form of dwarfism than the population at large. this is probably an example of

This is probably an example of "a founder effect".

Explanation:

The founder effect refers to a phenomena that arises when a small group of people turn out to be isolated from a larger group of people. The reason behind the founder effect is that randomness that go with choosing a minor group from a superior population.

There are many diseases in Amish which are inherited genetically, the reason is that their marriages are within their own community and this stops the entrance of new genetic variation.

answer; migrate from a region;

its a hope it good luck