What is the probability that Charles and Elaine will have a baby with cystic fibrosis? (Neither Charles nor Elaine has the disease)
The Probability that Elaine is a carrier is 2/3 (She does not have Cystic Fibrosis which eliminates one of the 4 possibilities. She does have 2 chances out of three of being a carrier. [Nn, nN (carriers) or NN]
The probability that the baby (?) has the disease (if Elaine is a carrier) is 1/4
The total probability is 2/3 x 1/4 or 1/6.