This paper provides an overview of the principal findings of recent Inter-university Population Research Centre studies of the Northeast part of Quebec province (Canada), and particularly the Saguenay region. These studies may be classified as the social history of population, historical demography, and population genetics. The purpose of the research is to study the population dynamics at the regional level from the 18th to the 20th centuries, to examine the structure and evolution of the gene pool. Focus is place on several defective genes, which cause very severe diseases in those regions of Quebec. The authors illustrate how population studies as such can help genetic epidemiology and preventive actions in general as well as ethical and legal issues raised by an approach relying on the use of a population register.