Edit: a question occurred to me. You might know the answer. I'm Irish and we have a big problem with cistic fibrosis here, it seems to be quite prevalent in our gene pool. If one is a carrier of the gene for CF, but don't have CF, could one still be affected by carrying the gene? In other words, maybe you or I don't have CF but by carrying one of the genes for it we might have mucus/sinus problems as a result?
Edit2 : apparently not.
"If you are a CF carrier, you do not have to worry that it will someday turn into CF disease. This can never happen because you have one normal CFTR gene. Being a CF carrier will not cause you to be ill or require you to seek treatment."
Still, I wonder....