 Once the tools of molecular genetics were in place, for each case the genetic difference correlated with the phenotypic difference could be located in the DNA and hence the genes of classical genetics could be molecularized (Waters 1994). The instrumental gene (as a unit that correlates with a phenotypic difference, segregates in a Mendelian manner, and is subject to recombination events) can remain indeterminate regarding DNA-sequence and need not correspond to a gene that has an identifiable product. In many cases one of the genetic variants consists in the absence or lack of a function of a gene product and there are many ways in which a change in DNA can result in the lack of a resource (Moss 2003: Ch. 1). The instrumental gene can correspond to elements of various size, including whole chromosome aberrations, and they might be located in coding regions as much as in regulatory elements or even insulator regions that only function to separate other functional elements, as long as a difference in  the sequence is correlated with a phenotypic effect (Griffiths &amp; Stotz 2006).