Actually, rexes can molt very extremely, until they pretty much have no hair. It comes back in after a little while, usually over a period of a few weeks. My little girl Georgia was molting when I got her, but her hair had just thinned out.

Then there are patchworks(I don't believe the double-rex thing). Here's a picture of a patchwork girlie. The Patchworks are so neat. The get patches of hair, and pretty much every week, lose it, and hair comes in in totally different places.
http://www.rattiusmaximus.com/twiste...er-of-rmax.htm
Another patchwork, Hairy:
http://www.freewebs.com/crittercity/...rldisafugs.htm

My hairless, bred from a hairlessxhairless combo, are nekkid, but get stray hairs on their thighs, tailhead, and other places, but they fall out the next day or so. They aren't true hairless as they have whiskers and eyebrow hairs. They were born, grew a thin coat of hair in, and then over a few weeks, it slowly was lost, coming out in patches.

Here are what hairless babies, from two non-hairless parents that carry the gene, look like compared to their siblings that are normal.
http://www.rattiusmaximus.com/MerleXMonchichi.htm

This is just what I've encountered with the hairless, patchwork and rex ratties. If anyone has any questions about any of the coat types, just ask, and I'll pass it along to Chrissy Knight, a friend and rat breeder. Her site is www.rattiusmaximus.com if you want to go look at all of the CUTE babies