Generally, the mutants are artificially grown clones taken from "Pure" Kaled DNA.  Sometimes they're spliced with genetic material from other races or, as monkeycloud mentioned, grown from tampered human DNA.  The Emperor who survived the Time War recreated a Dalek army by using human DNA which he "enhanced" to become (his words) "Pure and Blessed Dalek".  But then he was insane.  Even by Dalek standards, he was insane.

There were other Time War survivors; four Daleks known as the Cult of Skaro.  They were a sort of elite team, responsible for initiating original ideas (usually strongly discouraged) and radical ways to improve the race.  The four of them were most likely "pure" but Sec, their nominal leader, attempted to create a Human-Dalek hybrid with his own DNA.  The resulting humanoid creature (still with Sec's consciousness) was killed by the other members of the cult for being impure.  The remaining three Daleks battled with the Doctor and died, with the exception of Caan, who time jumped (Emergency Temporal Shift) away.

Caan reappeared later as an insane prophet with the ability to see all of time.  He engineered the destruction of the Daleks and their creator, Davros, because he realized they were an evil blight on the universe.  Things are never what they seem, however, and the Daleks reappeared later in a new form.  There's also a chance Caan and Davros survived, too. 

I know far too much about this subject, don't I?  :) 

Coppula eam se non posit acceptera jocularum.

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