How many phenotypes from one genotype? The case of Prion diseases

The usual assumption, namely that the underlying biochemical reactions in an organism tend to a unique steady-state, is shown to be not always correct. There are certain pathway mechanisms (e.g. positive feedback) which allow the system to exists in two alternative stable steady states. This bistability implies that environmental perturbations can "switch" the system from either state to the other. Such a switch takes place at the metabolic level and hence a single genotype can display two different, alternative, phenotypes without involving any changes in gene expression. The infective transmission of Scrapie-type diseases is explained here by such a mechanism involving protein-only changes.