I have no formal training in computer science as I have not yet taken any such classes, so perhaps this question appears naive. I was reading about BPP and it was claimed that a deterministic Turing machine is a special case of a probabilistic Turing machine. I don't understand why this is.
Deterministic turing machines have one transition at any given time. Nondeterministic machines are allowed to have multiple transitions out of a given state but can have just a single. Probabilistic turing machines pick one of the possible transitions and perform it based on a probability distribution. So if you make a deterministic turing machine then it is also a probabilistic turing machine where there is only ever one transition to choose from at any given time.
Probabilistic Turing machines are similar to deterministic Turing machines, but have the additional power of tossing coins. If you never make use of this power, you get a deterministic Turing machine.
In a similar fashion, a non-deterministic Turing machine is allowed to make guesses, but if it doesn't it is just a deterministic Turing machine.