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The question as to what extent a relationship between the genotype and the phenotype exists addresses the problem of the genetic determination of phenotypic characters. This problem would be straightforward if the respective fraction of the genome that contributes to the manifestation of a particular trait could be defined. However, approaches to identify this fraction have to reckon with complexities met at the genotypic as well as at the phenotypic level. Multiple phenotypic effects of one single gene or mutation are referred to as pleiotropy, and polygeny addresses the contribution of several genes to the manifestion of a phenotypic trait. Indeed, there is increasing evidence that almost each gene is pleiotropic and almost each character is polygenic. Evidently, the genetic contribution to the phenotype is rather complex. This complexity reflects the evolutionary past of the respective genes, and therefore the causes for it are to be sought in evolution. As Duboule and Wilkins (1998) stated: "The conventional view of single, dedicated gene function is not only erroneous in itself but subtly distorts thinking about the nature of the evolutionary process". Obviously, tracing the evolution of genes is expected to be the key to closer understanding their roles during the process of forming a phenotype. According to François Jacob (1977), the evolution of genes is characterized by what he calls "evolutionary tinkering" (or in his original term "bricolage").

DOI: 10.1007/978-3-642-50023-7_8

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