In the course of doing the quest for the woman in Megaton to write about a survival book I took over 600 points of radiation and came down with Critical Rad Sickness.
After she healed me it gave me a message saying I’d mutated and anytime a limb was critically damage I would begin to grow it back. I wonder if you can mutate yourself any other way…
Also, I found Dogmeat and he joined me! He’s so cute. 