Inbreeding in Gredos mountain range (Spain): Contribution of multiple consanguinity and intervalley variation

The present paper examines consanguineous marriages occurring between 1874 and 1975 in three valleys (Tormes, Alberche, and Tietar) in the Sierra de G redos mountain range, Avila province, Spain. Information was obtained from parish registers of 42 localities, corresponding to a total of 41,696 weddi ngs. Consanguineous marriages were defined as those up to the third degree of consanguinity (second cousins). From 1874 to 1975 the percentage of rela ted mates was 4.45% and the inbreeding coefficient was 0.0011868 (for 1874 to 1917 corresponding figures up to the fourth degree were 16.44% and 0.001 9085, respectively). In order to ascertain the characteristics and evolutio n of mating patterns in Gredos, the contribution of each degree of kinship was analyzed as a whole and then for each valley separately. Regarding tota l consanguineous marriages in Gredos, there is a low frequency of uncle-nie ce matings (0.21 %) and a first-second cousin mating ratio (C22/C33) of 0.2 3 (up to the third degree of consanguinity). Before 1918 multiple matings ( i.e., those involving more than a single relationship) accounted for 19.16% of consanguineous marriages (up to the fourth degree). The observed freque ncies of multiple consanguineous marriages was, on average. about twice tha t expected at random, and the proportion of such marriages to total inbreed ing was 34.65%. The temporal change of the Gredos inbreeding pattern was ch aracterized by a recent decrease; the highest inbreeding levels correspond to the period from 1915 to 1944. Finally, intervalley differences (maximum inbreeding coefficient in the Tormes, minimum in the Tietar) are interprete d considering the geography, population size, and population mobility for e ach valley.