Purity

The Thirteenth Doctor observed that the Daleks were "pretty obsessed" with purity, ironically for a species born out of mutation. (TV: Revolution of the Daleks) The Eighth Doctor noted that the pitfall of such a belief was that different factions of Daleks had different definitions of purity. (AUDIO: Restoration of the Daleks)

The Daleks of Skaro deemed a new race of what they called Mutant Daleks, created by Professor Martez on Red Rocket Rising, to represent a "deviation from the purity of the Dalek bloodline" and so saw to their extermination. (AUDIO: Blood of the Daleks)

As the Last Great Time War persisted, the Dalek ideals of racial purity were weakened. A Brief History of Time Lords claimed the Cult of Skaro were a result of this, (PROSE: A Brief History of Time Lords) though the members of the cult were known to be "pure" Daleks. (TV: Daleks in Manhattan) A group of the Dalek Scientific Division sought to restore the Daleks to a humanoid form, ignoring orders to stop over concerns of purity. (AUDIO: A Thing of Guile)

The Dalek Emperor in the Last Great Time War insisted that his Dalek of human origin were not "half-human", an observation from Rose Tyler which he found blasphemous. He claimed that everything human had been purged from them, that he had "cultivated pure and blessed Dalek." (TV: The Parting of the Ways)

Before their near-extinction at the end of the Time War, the Daleks created thousands of Progenitors containing pure Dalek DNA. A Dalek ship found the last remaining Progenitor, which deemed them to be impure, forcing them to goad the Eleventh Doctor into providing his testimony, verifying them as Dalek and so enabling the Progenitor to create a New Dalek Paradigm, the first members of which disintegrated their "inferior" and "unclean" predecessors who accepted their extermination. (TV: Victory of the Daleks)