  Sahotra  Sarkar’s (1998) detailed discussion of reductionism and genetics offers several relevant conceptual clarifications. Focusing on substantive issues about reduction in genetics (especially representation, see  Section 4.3),  rather than formal considerations in a theory of reduction, Sarkar develops a view of explanatory reduction that is not committed to a particular account of explanation. Two such basic types of reduction are “abstract hierarchical reduction” and “strong reduction” (or spatial hierarchical reduction), where the latter—unlike abstract hierarchical reduction—requires that entities of a lower hierarchical level are physical parts of higher level entities. (Each type of reduction can come in stronger and weaker forms as different kinds of approximations are involved in different explanations.) Sarkar argues that explanations in classical genetics, which explain phenotypic patterns in terms of classical genes, are abstract hierarchical reductions. The entities involved in this case are alleles, loci, linkage groups, genotypes, and phenotypes, which stand in hierarchical relations (e.g., alleles A and a form genotype Aa), with individual alleles occupying the lowest level. However, this is an abstract rather than a spatial hierarchy; the assumption that A and a map to Aa at the next higher level does not require Aa to be a physical entity that has A and a as its spatial parts. Furthermore, the genotypic entities in this hierarchy are governed by various transformations and regularities (principles of inheritance), such as segregation, linkage, and mutation, which help to explain phenotypic patterns of inheritance. While this abstract hierarchy can be interpreted as a spatial hierarchy by viewing classical genes as being parts of chromosomes, Sarkar’s point is that the various practices in classical genetics, such as segregation and linkage analysis, did not require a physical interpretation of genetic maps, and that explanations in terms of an abstract hierarchy can be genuine reductions since they invoke a lower and thus more fundamental hierarchical level.