 Moreover, even if the aim is to explain a feature of a single organism, it does not follow that an explanation should appeal exclusively to factors on the lowest level. An explanation should only include factors that are explanatorily relevant; an irrelevant factor is one whose omission from (or modification in) the explanation does not prevent the explanandum from following (Strevens 2008). This also holds for mechanistic explanation, which should cite those features as components of a mechanism whose modification would make a causal difference to the explanandum phenomenon (Craver 2007). For instance, chromosomes are relevant parts of the mechanisms needed to explain Mendelian patterns of inheritance (Darden 2005), but this particular explanation will apply regardless of the lower level, molecular composition of the chromosomes (e.g., whether or not the various genes have similar or very different DNA sequences).