Check out the AFRMA page on rex rats. Here's a quote for you, "Breeding Rex Rats
The Rex characteristic in rats is somewhat unusual in that it is dominant (I will put a qualifier on this and say that the Rex rats we commonly have in the fancy are dominant Rex).

This means that if you breed a Rex rat to any standard coated rat, half the babies in the litter will be Rex (if the genetics dice roll as they should). This makes it easy to produce more Rex babies. Keep in mind that standard coated rats produced out of Rex breedings do not carry Rex. Because Rex is a dominant gene, if it is there, you can see it!"