Familial Mediterranean fever (FMF) is caused by mutations within the Mediterranean fever (MEFV) gene. These gain of function mutations lead to an increased activation of the inflammasome pyrin with a subsequent disproportional proinflammatory reaction. Classically, in FMF patients two pathogenic mutations affecting both alleles are found in the molecular genetic analysis; however, it is well known that the phenotype can also be caused either by mutations with lower penetrance or unknown significance. Furthermore, in a significant number of patients only one or even no MEFV mutations can be detected. Heterozygous mutation carriers who do not suffer from classical FMF, can also present with other signs of inflammation, e. g. subclinical increased inflammation markers, associated inflammatory diseases or unclassified symptoms. Thus, FMF does not follow a classical autosomal recessive inheritance and a variable gene dose effect has to be considered, which is furthermore modulated by other mostly unknown genetic variants and environmental factors. This article summarizes the broad spectrum of clinical presentations associated with MEFV mutations and analyzes the effect of the gene dose on the phenotypical expression. Furthermore, the impact of the molecular genetic analysis on the diagnostics of a patient and on the individualized management of the disease is discussed.