Jane and Peter have two healthy children but Jane’s sister has a child recently diagnosed with CF. Peter has no family history of CF. What is the chance that Jane and Peter’s next child will have CF? Both of them are of Caucasian ancestry.
| Carrier | Non Carrier | |
| Prior | 2/3 | 1/3 |
| Conditional | 1/4 x 1/4 = 1/16 | 1 |
| Joint | 1/24 | 1/3 |
| Posterior | 1/9 | 8/9 |
The probability of Jane being a carrier of CF gene is 1/9.
The probability of Peter of being a carrier of CF gene based on ethnicity is 1/28
Then, multiply:
1/9 x 1/28 = 1/252 (the probability of Jane and Peter's third child to have CF)