Three important dominantly-inherited neurological diseases were discovered to be particularly common among various Jewish ethnic groups. For idiopathic torsion dystonia (ITD), previously thought to be recessively transmitted among Ashkenazi Jews, we have established an autosomal dominant mode of inheritance. This finding resulted from a country-wide survey of ITD in Israel and its subsequent genetic analysis. In Creutzfeldt-Jakob disease (CJD) the focus among Libyan Jews was previously thought to be related to culinary habits; a point mutation of the prion-protein gene was first described by us. The factors leading to the preservation of these disadvantageous genes in the communities are however unknown. Genetic analysis shows reduced penetrance in ITD and age-related expression in CJD. Factors leading to the expression of the genes were looked for. Data for ITD suggest that laterality of onset depends on motor dominance. Data on anticipation are presented. Factors possibly involved in the expression of the CJD codon 200 mutation are also discussed. A focus of myotonic dystrophy was documented among Yemenite Jews. The preservation of this disadvantageous, dominantly-inherited gene (which leads to diminished reproductive abilities), was found to be social rather than biological, related to reduced age of marriage and number of offspring in this ethnic group. These data show an interaction between genetic and other biological and external factors in the expression of these three diseases.