Mr & Mrs Smith have a child who has Cystic Fibrosis, yet they do not personally suffer from the condition. Explain, with the use of genetic crosses, how this is possible and, if they had another child, how likely the second child will have Cystic Fibrosis

As neither of the parents have the condition, it therefore is assumed to be a Homozygous Recessive condition, so requires both the recessive alleles for the disease to be present in an individual (eg ff). As the parents do no suffer, yet hold the allele to pass on the condition, they must be carriers (Ff). By drawing out the genetic grid, you can see that, by both parents being carriers, they have the possibility to have offspring with the homozygous dominant (FF), heterozygous carriers (Ff) and the homozygous recessive (ff), the latter will cause the disease.
As for how likely it is that a second child will also have the condition, from the genetic cross, you make 1 x FF, 2 x Ff and 1 x ff. Therefore there are 4 possible combinations, and one of those will be the homozygous recessive, ff. Therefore there is a 25% chance of the second offspring having the condition.