Recent approaches to the new genetics stress the importance of placing hereditary risk within the context of familial beliefs and dynamics. Nonetheless, few empirical studies on predictive genetic testing have explored the meaning and significance of hereditary risk within everyday life. Drawing upon in-depth interviews with 21 families, this paper examines the social construction of hereditary risk for one adult onset disorder, Huntington Disease. Highlighting the social, biographical and temporal factors that families consider when discussing risk and its modification through predictive genetic testing, we find that Mendelian theories of inheritance seldom provide an adequate framework. Such objectified knowledge makes sense on an abstract level but is ultimately inadequate for describing the fluctuating relevance of risk as it develops within the nexus of familial relations.