Final answer:
The high incidence of rare genetic diseases among the Amish and French-speaking Quebecois is due to the founder effect, where a small initial population with limited genetic diversity leads to a higher prevalence of certain genetic traits due to genetic drift and practices of endogamy.
Step-by-step explanation:
The high incidence of rare genetic diseases in populations like the Amish in Pennsylvania or French-speaking people in Quebec is primarily due to the founder effect. This effect occurs when a new population is established by a small number of individuals, carrying a limited genetic variation from the larger original population. For example, the Amish population stems from about 200 German immigrants, one of whom carried a recessive allele for Ellis-van Creveld syndrome, a type of dwarfism characterized by extra fingers and short limbs. Due to the limited genetic pool and practices like endogamy, this allele became more prevalent within the Amish community, leading to a higher incidence of the syndrome today. Genetic isolation and the founder effect have similarly impacted the French Canadians with certain genetic disorders becoming more common due to a narrowed genetic diversity and endogamy, reinforcing the effects of genetic drift.