INTERNATIONAL JOURNAL OFANTHROPOLOGY Vol.20 - n. 3-4 (259-275)- 2005 Marin A. Dipartimento di Biologia Animale e dell'Uomo, Universitg~di Torino Achilli A. Dipartimento di Genetica e Microbiologia, Universitgl di Pavia Di Gaetano C. Dipartimento Genetica, Biologia e Biochimica, Universitgzdi Torino Guarrera S. Dipartimento Genetica, Biologia e Biochimica, Universitg~di Torino Rengo C. Dipartimento di Genetica e Microbiologia, Universith di Pavia Torroni A. Dipartimento di Genetica e Microbiologia, Universitgt di Pavia Piazza A. Dipartimento Genetica, Biologia e Biochimica, Universith di Torino Bo~tsch G. UMR 6578 du CNRS, Facult~ de Mddecine, Universit~ de la M~diterrange, Marseille, France Sella G. Dipartimento di Biologia Animale e dell'Uomo, Universitd di Torino Rabino Massa E. Dipartimento di Biologia Animale e dell'Uomo, Universit&di Torino Keywords: mtDNA, Y Chromosome, biodemography, isolated population. Biodemographic and molecular analysis of an isolated Alpine population (Postua) Isolated populations have been the object of several genetic and anthropological studies, since endogamy and inbreeding often lead to the acquisition of a particular gene pool. In this context, we studied the small, ancient population of Postua in the north-western Italian Alps. We used biodemo- graphic and molecular techniques to analyse the population structure in order to evaluate the relationship between geo- graphical and genetic isolation. We examined about 26,000 certificates kept in the town and parish archives, concerning the period from 1640 to 1999. High rates of endogamy and isonymy, short marriage dis- tances and a low ratio between the number of surnames and the number of individuals were inferred. In the molecular analysis, we compared the distribution of Y chromosome SNPs (single nucleotide polymorphisms) with those of mitochondrial variations and Y chromosomal microsatellites (short tandem repeat polymorphisms) in 102 healthy individuals originating from Postua. A control sam- ple (94 individuals) was collected from a plain area, 50 km away. We examined 23 SNPs and an Aiu repeat, located in the non- recombinant portion of the Y chromosome. To further delin- eate Y chromosome lineages, the biallelic haplogroups were further resolved using Y microsatellite markers (DYS19, DYS391, DYS392, DYS393). Mitochondrial HVS-I and HVS-II regions were sequenced, and RFLP screening with the six classical enzymes was per- formed. Postua is similar to other populations living in northern Italy, but it shows a lower number of haplotypes. The samples were compared with other European popula- tions. We calculated genetic distances according to Reynold and Nei and we carried out a phylogenetic analysis by phy- iogenetic trees and reduced median networks construction. Postua clusters with other samples from northern Italy but in a separate position, probably indicating drift phenomena. These relationships are supported by AMOVA (analysis of molecular variance). Our results suggest that the influence of neighbouring popu- lations on the gene pool of Postua has been very low through both females and males.