Fragile X families in a northern Swedish county--a genealogical study demonstrating apparent paternal transmission from the 18th century.

Eleven families including 35 cases with fra(X) mental retardation (MR) were traced genealogically using the Research Archives at Umeå University. Seven of the cases were women with fra(X). All of the families originated partly or totally from the county of Västerbotten. It was possible to link 7 of the index families to common ancestors over an 8-11 generation span. The remaining 4 families were not traced to the same ancestors. However, they were linked together pair-wise over a 7-8 generation span. Transmission of the fra(X) mutation was studied in these families. In the pedigree analyses, priority was given to maternal transmission. In 2 families the fra(X) mutation was transmitted solely through females over 7 or 8 generations respectively. Within 9 families the mutation was transmitted by males in 2-5 generations in order to reach common ancestors.