Cystic fibrosis is a recessive condition; meaning if a dominant condition takes over it will not be seen in the offspring. The man genotype with cystic fibrosis can only be this: cc (when c is equal to the cystic fibrosis and C is equal to not having one). The woman with no history of cystic fibrosis can possibly have either of these two genotypes: Cc or CC.
To see their offspring’s possible genotype, we’ll use a punnett square let’s start with Cc:
C c
c Cc cc
c Cc cc There is a 50 % chance of the child having cystic fibrosis.
To see their offsprings possible genotype with CC:
C C
c Cc Cc
c Cc Cc There is no chance that the child will have cystic fibrosis.