  Classical genetics nevertheless finds a simple way to explain transmission phenomena by appealing to the difference principle, according to which particular differences in particular genes cause particular differences in phenotypic traits in particular contexts (see section 2.1). Sturtevant alludes to this principle in the first sentence of the quotation above and again in the emphasized clause. So the question arises, can this relationship be captured at the molecular level? And the answer is yes. The differences used by classical geneticists to explain inheritance patterns have been routinely identified at the molecular level by contemporary geneticists.