Recessive because 2 of these alleles must be present to produce a child with cystic fibrosis. When only 1 parent is a cf carrier that allele wont be passed on because the dominant allele will be prominent. If 1 parent is a suffer of cf then all offspring will be a cf carrier and have the cf recessive allele. But when 2 parents with this allele have children then the chances of there child having cf increases to a 1 in 4 chance.
The allele for CF is recessive, which means that the child has to inherit a CF gene from both parents who are carriers and have no idea of their mutation. Interestingly when two first cousins marry their children have 8 times more chance of being a CF sufferer than if either parent had out married. This is because the same shared grandparent of the cousins could be a possible carrier and has handed it down to both his/her grandchild.