How to calculate probability of combinations of not all possible combination is possible?
For example, given a test that determines the gender of a fetus with 90% accuracy. If somebody takes the test twice, and both said "a boy", what is the probability that it's actually a boy? Using naive coin toss calculation, the probability of both test being accurate is 0.9*0.9 = 0.81, but the possibility of both being false is 0.1*0.1 = 0.01, which doesn't add up to 1 because the outcome where only one of the tests is wrong is impossible in this case.
So, how to calculate probability in this case?