Gene Action and Genetic Strategies. How do genes manage to determine the development and function of the organism? This is the basic problem of genetic biology that should be solved. The topics discussed earlier such as the structure of the genetic material and Mendelian segregation are interesting scientifically because they help in solving this basic question. In Chap.6 Mendelism is described as the central paradigm of genetics. Thus, a Mendelian mode of inheritance for a certain phenotype points to a specific change within the information-carrying DNA, and the study of linkage with DNA variants as markers leads to precise localization of the gene causing this phenotype. After the nature of the genetic alteration has been defined, the path of gene action between genotype and phenotype can be elucidated. For a long time linkage studies were known in principle to be the “King’s road” leading to gene localization. Linkage study in humans is more difficult than in experimental organisms because of the lack of directed matings. Although statistical techniques needed for human gene mapping have been available for some time, practical implementation was impeded by the lack of suitable genetic markers that could be used as landmarks for gene localization. This has fundamentally changed with discovery of the very frequent DNA polymorphisms. (Sect.12.1.2).