 It remained central to experiments involving crosses between lines that, as much as possible, those lines were inbred and identical and homozygous for genes influencing all traits apart from for the traits under study. While the identical homozygous genes might have an influence on the development of any focal trait, differences in that trait could be attributed to differences in the genes that were not identical among the crossed lines. (Indeed, by the 1930s heredity had come to refer to the transmission of and cross-generational patterns in these differences, not to the development of the similarities from which differences depart [Sapp 1987].) Genotype could be applied to classes of organisms with a specific pair of genes (or small set of pairs) or to the specific pairs of genes themselves (matching the connotation of type as an abstraction away from the full set of observed characteristics). In Mendelian experiments phenotypes-as-classes demarcated by a small set of traits could be used to identify genotypes-as-classes. Then, once the genotype as pair(s) of genes was mapped to a locus on the chromosome, the direction could be reversed: the phenotype would then be the subset of an organism’s traits associated with the genotype under given conditions (a forerunner here of the predominant current-day meaning).