The gene responsible for a rats ears is du (If I am remembering right anyway). Standard ears are dominant,while dumbo is recessive. A standard eared rat would be dudu (one Du gene from reach parent). A dumbo eared rat would be dudu (one recessive gene from each parent).
Since the du fene is recessive, if you breed dudu to dudu, you will get all dudu (dumbo). If you breed Dudu to Dudu, you will get all standard eared (dudu). Where it gets tricky is if you are breeding dumbo carriers..
Say you breed a standard eared rat (Dudu), to a dumbo carrier (which will appear to be standard, but is genetically different-Dudu. One dominant gene, one recessive). You would get an entire litter of standard eared rats, half of which (in theory) would carry