 Mendelian experiments require further control than for inbred lines, because the lines have to be raised in uniform conditions, crossed, and self-pollinated. Phenotypes can then be used to discriminate among multiple hypotheses about genotypes, where the phenotype is a class of organisms that share only some part of the whole set of the organisms’ traits and the genotype is a class or organisms that have some part of their germ cells in common. The relevant part of the genotype was shown to be pairs of genes located along chromosomes as long as, given the control entailed by Mendelian experiments, the focus lay on differences in traits, not on how an offspring develops to have the trait at all. Recall, as Johannsen noted, that Mendelian experiments are limited in examining the species-typical aspects of the germ cells and subsequent development. Again, a program for reintegrating what is abstracted away through experimental control can be imagined: researchers identify the material constituents of the genotype and then trace how all these constituents influence the development over time of the species-typical traits. From the composite of these influences the organism as a structured whole might emerge. Two emerging features of the study of heredity, however, work against such a reintegration program: Heredity, as mentioned earlier, has become equated with the transmission of and cross-generational patterns in the differences. That means development became a separate and secondary matter (Sapp 1987); analysis of the dynamics of species-typical development of morphological structure was eclipsed by genetics.