Jason knows that he is the carrier of CF. His wife, Julianne, has brother with CF, which means probability is 2/3 that she is carrier. If Julianne is carrier, each child she has with Jason has probability of 1/4 of having CF. If she is not carrier, her children can't have disease. Jason and Julianne have one child, who doesn't have CF. This information decreases the probability that Julianne is carrier. Use Bayes' rule to find conditional probability which Julianne is carrier, given that she and Jason have one child who doesn't have CF.