There is some evidence that the genetic constitution or predisposition of an individual has influence on the probability to sicken from cancer and other diseases. Mutations that are associated with disease-predisposition, can either occur spontaneously, i.e. somatically, or they are inherited. Congenital diseases are roughly divided into the following three different categories:

• Chromosomal diseases caused by changes in number or structure of chromosomes (e.g. Down-syndrome).

• Monogenic changes are caused by a single aberrated gene. Up today, about 4000 monogenic diseases - mainly metabolic diseases - are described, e.g. mucoviscidosis, Huntignton's disease, Marfan syndrome, and the Li-Fraumeni syndrome.

• Cardiovascular diseases, cancer and other common diseases seem to be caused by a complex interaction between predisposition, environmental effects, and ageing and are multifactorial and polygenic (more than one gene is involved in disease process). Multifactorial genes can influence the progression of infectious diseases or the sensibility to mutagens like cigarette smoke. Examples for multifactorial diseases are pylorus stenosis, hip dysplasia, or allergies.