In order to evaluate the genetic epidemiology of deafness in Turkey, we first analyzed the pedigree data obtained from 2,169 families whose children were students of the schools for hearing loss/deafness in 31 cities of Turkey. Single major locus segregation analysis was performed after families were grouped according to hearing status of the parents. The results showed that sporadic phenocopies, autosomal dominant, and autosomal recessive transmission account for 18.2%, 4.9%, and 76.9% of the cases respectively, after exclusion of probands with unequivocal evidence for environmental etiologies. The high frequency of autosomal recessive transmission Of this study differs front those of previous ones in Western populations. We subsequently analyzed the data from a subset of 574 unrelated families that were evaluated clinically, including mutation analysis of the GJB2 gene in 406 probands. Biallelic mutations were detected in 22.4% of all probands. They were present in 68.8% of probands whose parents were both deaf, yet in only 9.3% when both parents were hearing and consanguineous without a family history of deafness. Our study shows that GJB2 is the major gene for deafness in Turkey and was amplified in deaf by deaf matings, since assortative mating preferentially affects common genes. Deafness in the remaining families appears to result from mutations at many loci that are less frequent causes of deafness, because consanguinity has a proportionally greater effect on rare genes. Conclusions of this study may be relevant to other populations where consanguineous or assortative mating is present with various frequencies. (C) 2007 Wiley-Liss, Inc.