The influence of past endogamy and consanguinity on genetic disorders in northern Sweden.

It has been widely believed that consanguineous marriage was infrequent in northern Europe. As part of ongoing studies into the population structure of northern Sweden, the Demographic DataBase of Umeå University has undertaken digitization of the parish record books of the Swedish Lutheran Church, which date back to the late 17th century. To examine the prevalence and patterns of consanguineous marriage, information from the DataBase was abstracted for the Skellefteå region during the period 1720-1899 and extended family pedigrees constructed. Of the 14,639 marriages recorded, 3,043 (20.8%) were between couples related as sixth cousins or closer. Following changes in the Swedish civil law in 1844 that removed the requirement of royal dispensation for first cousin unions, a significant increase in first cousin marriages occurred during the next two generations, even though the total population of the region grew significantly. There was also strong evidence that consanguineous marriages were favoured within particular families. The findings of the study are consistent with the patterns of single gene disorders reported in specific communities in the region, and they suggest that founder effect, drift and consanguinity all were important influences on population genetic structure in previous generations.