Interactions between genotype and phenotype in bleeding and thrombosis.

The hemostatic balance is the result of an equilibrium between procoagulant and anticoagulant factors that interact with each other to ensure hemostasis at sites of vascular injury. Abnormalities of hemostatic factors due to defects in the corresponding genes can result in a tendency to hemorrhage or thrombosis. In the last few decades, the progressive identification of the mechanisms underlying coagulation disorders led to the awareness that clinical phenotypes are only rarely the result of single gene defects but are more often influenced by multiple factors. Interactions between different genes or between genes and other acquired factors may account for the phenotypic variability of most coagulation disorders, by improving or worsening their clinical manifestations.1 Examples of interactions between genotype and phenotype in both hemorrhagic and thrombotic disorders are given in this article.