 Not all aspects of the study of heredity could be made an experimental endeavor through Mendelian methods. There were many traits for which the continuous variation could not be subdivided into discrete phenotypes, let alone linked to genotypes, especially for traits in agriculture of economic interest such as yield of plant and animal varieties or breeds. By the end of the 1910s Ronald Fisher and Sewall Wright had begun to address the need to reconcile the discreteness of genotypes with continuous variation in many observable traits. In the mathematical models of a field that came to be known as quantitative genetics, differences between unobserved theoretical genotypes (in the sense of pairs of genes) at each of a large number of loci contribute to differences in the trait, modulated by degrees of correspondingly theoretical dominance and epistasis. Under the reasonable assumption that more of the genes are shared among relatives than in the population as a whole, data on a given trait as it varies across genealogically defined lines or groups of specified relatedness could be analyzed so as to provide predictions of changes in the average value of the trait in the population under selective breeding. Of course, the trait values and thus the predictions depended on the conditions in which the organisms developed, but in the laboratory and, to varying degrees, in agricultural breeding, conditions could be replicated. For the breeder, the focus of the quantitative genetic data analysis on differences in the trait makes practical sense; it is not necessary to know the mechanisms through which the traits developed as organisms reacted to conditions. In other words, the meanings of genotype, phenotype, and their distinction again make sense as an abstraction through practices of control over biological materials and conditions in agricultural and laboratory breeding and the allied use of models and analysis of data.