Evolution and its Mechanisms

Pick and a rare genetic disorder that is common among the Amish and tell me how common the disease is around the world vs among the Amish? Give me an estimated percentage of afflicted individuals! (I don't need to know details of the genetic disease just its name). The important idea is to understand why disease-associated alleles are found in high frequencies in the first place!
Explain in evolutionary terms why the genetic disorder that you investigated is found in unusually high frequencies among the Amish in the first place. You will need to give me one evolutionary mechanism that can specifically explain the Amish issue. Make sure you name the evolutionary mechanism and give a quick explanation of how the evolutionary mechanism works.
Explain how the Amish might be able to curtail the incidence of your chosen genetic diseases using evolutionary knowledge. You will need to give me one evolutionary mechanism that would be able to fix the Amish dilemma. Make sure name the evolutionary mechanism and give a quick explanation of how the evolutionary mechanism would be able to work.
Lastly, remember to cite all your work!