familial dysautonomia , the riley - day syndrome , is an autosomal recessive disorder characterized by developmental loss of neurons from the sensory and autonomic nervous system . it is limited to the ashkenazi jewish population , where the carrier frequency is 1 in 30 . we have mapped the fd gene to the chromosome region 9q31 - q33 by linkage with ten dna markers in twenty - six families . the maximum lod score of 21 . 1 with no recombinants was achieved with d9s58 . this marker also showed strong linkage disequilibrium with fd , with one allele present on 73 % of all affected chromosomes compared to 5 . 4 % of control chromosomes . the other nine markers , distributed within 23 cm proximal or distal to d9s58 , also yielded significant linkage to fd . d9s53 and d9s105 represent the closest flanking markers for the disease gene . this localization will permit prenatal diagnosis of fd in affected families .