Past research had shown the use of pelage-based subspecies designations was questionable. Complete colour intergradation occurs in both thinhorn sheep subspecies (i.e., Dall's and Stone's), ranging between white and dark morphs of the species. Intermediately coloured populations, called Fannin sheep were originally (incorrectly) identified as a unique subspecies ("O. d. fannini") with distributions inhabiting in the Pelly Mountains and Ogilvie Mountains of the Yukon Territory. Fannin sheep have more recently been confirmed as admixed individuals with predominantly Dall's sheep genetic origins. Previous mitochondrial DNA evidence had shown no molecular division along earlier subspecies boundaries, although evidence from nuclear DNA may provide some support. Current taxonomy using mitochondrial DNA information may be less reliable due to hybridization between "O. dalli" and "O. canadensis" recorded in evolutionary history.