Are you sure that normal wasn't a low quality rex? I've run in to that a couple times with our rats. Just a few weeks ago I got a litter with all rexes in it from a non-rex male bred to what I thought was a non-rex female. After digging through the records, it turned out she was a rex, we had just forgotten.A lot of times female rexes will lose most of their curliness as they get older.