A family is affected with Huntington disease, an adult-onset neurologic disorder. Fifty percent (50%) of people who inherit the HD gene show some symptoms of HD by the age of 50. Mary is 50 and asymptomatic, but her father died of HD.
What is the probability that Mary’s son, Bob, inherited the HD gene?
| Carrier | Non Carrier | |
| Prior | 1/2 | 1/2 |
| Conditional | 1/2 | 1 |
| Joint | 1/4 | 1/2 |
| Posterior | 1/4 / 1/4 + 1/2 = 1/3 | 1/2 / 1/4 + 1/2 = 2/3 |
Mary's probability of being a carrier = 1/3
Mary's probability of having unaffected son = 1/2
1/3 x 1/2 = 1/6 (Bob's probability of inheriting the HD gene)