That was just a figure of expression. I mean catch, as in born with it. And slimmer, as in the chances of being born with it become less likely with every passing generation unless, like you said, somewhere along the line 2 parents, who both happen to have the CF gene, meet and have kids. But even if only one parent has the gene, there is always a possibility, however remote, that their kids could be born with the condition. When I was in college, I did an assignment about genetics, and I remember reading a book about albinos. It described a couple who were both albino, so reason would say that all their kids would be born albino, too. However, the book said the couple had at least 1 child who was not albino for some reason.