Medical researchers love genetic diseases like thalassemias and Factor VIII clotting deficiency because they are monogenetic - only one gene is involved, the inheritance is sex-linked or simple Mendelian in nature and the rules are well understood. Beyond that things get progressively more geometrically complicated.

Most cancers and other diseases involve a range of genetic levels and environmental influences interacting together over a long period of time. They will prove very difficult to disentangle.

Other processes might involve polymorphisms for genes that detoxify carcinogens, such as the cytochrome 450 system. Some polymorphisms may do so more effectively, thereby reducing exposure and hence risk of developing cancer.

There may be polymorphisms that specify receptor densities on neurones, affecting sensitivity to adrenaline. These may be important in people who take risks, thereby exposing themselves to accidents. Others may lead to people tolerating more psychoactive chemicals, such as alcohol or nicotine.

There are probably vastly more complex mechanisms to be discovered.