 The gene concept that structured the work of early Mendelism and chromosomal genetics is referred to as “instrumental gene” (Falk 1986; Griffiths &amp; Stotz 2006). Geneticists routinely identified genes based on phenotypic differences in hybridization experiments or the analysis of pedigrees (see  §1.2).  Differences between, first germ cells, and, later, more specifically, between chromosomes, had to be assumed to explain patterns of phenotypic correlation between sexually reproducing organisms and their offspring. These units of difference in the genetic material could also, and more importantly, be used to predict phenotypes. For these purposes, it was not necessary to know what exactly their physical location or material composition was. In this sense, the concept was instrumental. In a similar way, Lenny Moss (2003: Ch. 1) characterizes the gene-P as a gene defined by its phenotypic effect and used for prediction (the “P” can thus stand for “phenotype” or “prediction”, but Moss also points out that the concept is associated with instrumental preformationism).