Here is the scoop about Rough and Smooth coats on Collies. I am no long-time expert, but I also had trouble understanding this. This explanation is very simple and intended for the beginner. I hope it will answer your questions.
The Smooth coat is a dominant gene. In order for a dog to be Smooth it must carry the dominant smooth gene...if it does carry the gene it is Smooth.. There is no such thing as a recessive smooth gene.
If your dog is Rough it has no Smooth gene to pass on. The only way to get a Smooth from a Rough is to breed it to a Smooth. In that breeding, you will get some Smooth and some Rough..The Roughs again cannot produce a smooth, (the smooth gene is never a recessive remember) but the Smooths can produce Roughs as rough is carried as a recessive gene.
If you breed two Smooths together (both having a rough parent therefor carring a rough recessive gene..you will get some Smooths WITH a recessive rough gene that can produce roughs. It inherited one dominante smooth gene and one recessive rough gene ...and some Smooths that will only produce Smooths as they inherited two Smooth genes, one from each parent, they have no recessive rough gene so they will not produce a Rough no matter what you breed it too, And, you may get a Rough (inheriting two recessive rough genes). Even a rough from two smooth parents cannot produce a smooth. It is never a recessive.
The people that say they do not want to use a smooth in their breeding program because they don't want to lose their nice full coat factor do not understand the genetics of the coat factor. Using a rough from a smooth parent will in no way make the rough hair less long..it never mixes.
You don't have to worry about a smooth coming along later in your breeding program either as I have heard some say......IT IS NEVER RECESSIVE.
