The prevention of hereditary diseases after birth is essentially based on the detection of these diseases at their preclinical level. This may be made possible through a careful pedigree examination and possible identification of a "genealogical alarm", as well as a detailed clinical examination of the subject and possible identification of preclinical signs of the pathogenesis, i.e., of a "clinical alarm". The respective roles of chronogenetics and of gemellology in this approach are reviewed.