Ellis-van Creveld syndrome is an inherited condition affecting bone growth. The primary symptom is short stature. Other symptoms include abnormal numbers of finger and toes and tooth and nail abnormalities. Some individuals with the condition have heart abnormalities.
The frequency of the condition in most populations is low. However, the condition occurs at a much higher rate in the Amish population descended from a group who migrated from Europe to Pennsylvania in the 1700s.
Which of the following explains the difference between the Amish frequency of the condition and the frequency of the population they migrated from?