Trends in inbreeding, isonymy, and repeated pairs of surnames in the Valserine Valley (French Jura), 1763-1972

The estimates of inbreeding derived from pedigrees and frequency of isonymo us marriages (i.e., between persons of the same surname) are compared using genealogical and isonymic information from 4899 marriages recorded between 1763 and 1972 in 4 rural villages of the French Jura region (a mountainous area near the Swiss border). Before the second half of the 20th century, t he two kinds of estimates show a different temporal evolution. The mean inb reeding coefficient based on pedigrees increases between 1763 and 1852 and reaches a maximum between 1853 and 1882 (alpha = 0.0028), with a very low p ercentage (<1%) the result of remote kinship. The mean inbreeding coefficie nt based on isonymy is always higher, with a maximum observed between 1793 and 1822 (F = 0.0200), and it remains roughly the same between 1763 and 188 2 (F = 0.0150), with a high percentage resulting from a random component (F -r), a consequence of the small population size and genetic drift. After 18 83, the 2 mean coefficients decrease. This discordance is largely explained by the poor quality, for the first periods, of the genealogical data base, which ignores the more remote links of kinship, justifying the use of the model of Crow and Mange (1965) to explore consanguinity during the more anc ient periods. The temporal evolution of the repeated pairs of surnames inde x (RP) confirms the recent evolution of the marital structure of the valley . Moreover, it appears that isonymous marriages and repeated and unique pai rs of surnames constitute 3 distinct matrimonial groups characterized by bo th a different mean coefficient of inbreeding (alpha) and a different rate of endogamy.