Why does Nietzsche not commit the genetic fallacy, in his genealogy of morals?
There's at least a handful of articles on this, which I didn't yet read; I make here my 1st port of call.
Short answer: no.
Slightly longer explanation: committing the genetic fallacy depends not on using or developing a history, but on arguing that something is wrong based solely on its source. Something like: Kant's morality is wrong because Kant was a Prussian; Prussia no longer exists, and thus we can safely ignore it. Nietzsche' argument is quite a bit more complex and is based on showing that our current models of morality are based on a certain historical conception whose framework has been left aside without calling that framework into question. For example, Kant's morality is based on the promise of a "pure" reason, but really simply endorses a Christian conception of morality (I'm not saying this is exactly Nietzsche's argument; it's meant as an example that's closer to what Nietzsche is doing). In other words, for Nietzsche, our moral thinking hasn't really shed the history it claims it has overcome. What is needed is a more thorough-going criticism of our moral reasoning that ask not "what should we value", but "how should we value".