Anton Callaway reports here that “a molecular assay done by Oregon State University scientists showed that while it was related to ‘Magness’, ‘Warren’ is distinct (Karp. 2011).”
Callaway shares @clarkinks’ assessment of Magness’ greater fireblight susceptibility, and notes that Magness “is resistant to blight through the blossoms (the most common route of entry), but if the trunk is inoculated (in nature, this usually means a hailstorm), then it is susceptible. My tree produced several crops before a hailstorm hit when there was blight in the orchard and the tree quickly succumbed. The breeders also noticed this attribute.”
Edit: I tried to find that study, and the source Callaway relies on seems to be an article in the LA times about the Warren Pear that refers to unpublished enzyme analyses cited by Richard Bell.