For the identification and description of susceptibility genes for complex
diseases using genetic markers there are two basic strategies: genome scans
or the investigation of candidate genes. The designs can be quite differen
t for the two approaches and decide, more than in general epidemiology, the
methods that can be applied subsequently. Study design, recruitment and sa
mple size are discussed further in the context of the Transmission/Disequil
ibrium Test, a family-based simultaneous test for linkage and the context o
f the association.