Hi Elke,
What you are trying to do here is technically a pain in the whatsit even for seasoned breeders, and i find even more complicated than getting more recessive traits to throw.
There are basically 2 ways to go about it but i think as Dakilla says your best bet is to get along to the Kitty College classes where there is no such thing as a dumb question and the solutions will be explained to you as simply as possible, and also if one tries explaining the ins and outs to you here it may be more confusing than helpfull.
This will also be a very useful subject for others to learn from.
Also Dakilla has a good point about maybe something pretty recessive hiding somewhere so this could be well worth exploring too.
Good luck anyhow
