Abstract for presentation at 11th International Congress of Human Genetics

Remarkably few representative studies have been conducted into the prevalence and possible influence of close kin unions on fertility and mortality in northern European populations, in part because marriages between couples related up to first cousins (F=0.0625) are legal. The Demographic DataBase in Umeċ University provides a unique opportunity to correct this situation, with detailed information collected by the State Lutheran Church available on births, deaths and marriages in the Skellefteċ region of Sweden for the period 1720-1899. The data are made more interesting by the fact that until 1680 first cousin unions were prohibited in Sweden, and from 1680 until 1844 a royal dispensation was needed before such unions could proceed. Of the 14,639 marriages initially studied, 20.8% were between couples related as sixth cousins or closer (F=0.00204), with a significant increase in first cousin marriages post-1844. Using logistic regression, two subsets of marriages contracted from 1780 to 1899 onwards were investigated with respect to fertility and mortality. First cousin marriages were strongly favoured by freeholders and peasant landowning families. In some families they had been preferentially contracted in successive generations, resulting in multiple consanguinity pathways and cumulative coefficients of inbreeding up to F=0.14840. Consanguinity appeared to exert no influence on fertility. However, first cousin couples had significantly higher rates of stillbirths (p<0.01), and more deaths in infancy (p<0.01) and early childhood (p<0.01) among their progeny. This excess mortality appeared to be associated with the expression of detrimental recessive genes, with evidence for the clustering of multiple deaths within first cousin families consistent with a genetic aetiology. Overall, the data confirm the significance of consanguinity as an important contributory factor in the prevalence and patterns of disease in northern Sweden.