The use of parental isonymy in inbreeding in two Outer Hebridean populations.

The use of isonomic associations between the parents of spouses is described. Scottish marriage registers contain information on the natal surnames of all four parents of spouses and enable all the four possible types of isonymy between pairs of parents to be identified. Thus assumptions about the sex labelling of pedigrees may be tested and inbreeding coefficients calculated using either each type of isonymy singly or by pooling all types to give 'overall' coefficients. By this latter method the number of potential isonomous events is increased to four times the number of marriages. Assumptions about the randomness of migration with regard to sex may also be tested by comparing coefficients of relationship by isonymy calculated using either paternal or maternal natal surnames. Within populations the coefficients should be lower for the more migrant sex: between populations the reverse should be the case. Similarly the intergenerational 'fluxes' of surnames in parents of either sex may be compared as may be similar variation in surname frequencies. Two small Outer Hebridean islands, Scalpay and Berneray were used to test these assumptions. It was concluded that while the sex labelling of pedigrees appeared to be at random, there was considerable variation in the isonymy inbreeding coefficients and that because of the 4 X greater amount of data, 'overall' coefficients are more reliable. In both islands there appeared to be a greater lability in migrational terms of the female population, although it was impracticable to examine all marriages involving residents of the two islands.