 Regarding spatial hierarchical reduction, Sarkar argues that this regularly occurs in molecular biology, where certain cellular features are explained by molecular biology, more precisely macromolecular physics. The various components of cells form nested part-whole relations, resulting in a spatial hierarchy. Many mechanisms of classical genetics can be reductively explained by the physical principles governing the behavior of macromolecules (though approximations are still necessary). Explanatorily relevant principles for the physics of macromolecules include weak interactions, hydrogen bonding, hydrophobic bonding, allosteric transformations, lock-key fit, and the idea that structure determines function. Sarkar’s central tenet is that the elucidation of the molecular mechanisms of replication, recombination, and cell division provide reductive explanations of the principles of classical genetics. While classical genetics offered an account of gene transmission and could rely on cytological explanations, these accounts did not yield a complete explanation of how and why recombination occurs. Molecular biology filled in this gap and corrected classical genetics.