 In this tradition, often referred to as “classical genetics”, genes were hypothetical material units of the genetic material, located in the chromosomes. But unlike the material carriers of heredity assumed in the nineteenth century, they did not give rise to characters in a straightforward manner. Many-many relations between genes and characters became obvious early on. All characters were influenced by many genes (polygenic) and most genes affected many characters (pleiotropic). The only kind of causal statement that was justified was that a difference in a gene resulted in one or more differences on the phenotypic level (Schwartz 2000).