  The concept of actual difference making can be illustrated with the difference principle of classical genetics (section 2.1). According to this principle, genes can be difference makers with respect to phenotypic differences in particular genetic and environmental contexts. So, it identifies potential difference makers. When this principle is used to explain an actual hereditary pattern, it is applied to genes that actually differed in the population exhibiting the pattern (often an experimental population). In such cases, an actual difference in the gene among the organisms in the population caused the actual phenotypic differences in that population (see Gifford 1990). That is, the gene was the actual difference maker, not just a potential difference maker (in that population). 