deriving a taxonomy base of a taxonomy in a field type

another issue I'm struggling with is:

when a user fills out a registration forum, they are asked what area is your Profession,they are given a taxonomy from which they can choose a few, based on that choice I need to load another taxonomy/ies so they can better define there Profession.

ideally this would be done on the same page with a drop down menu that updates when the prior is selected.

Ideas/links to resources?

Thanks,
Tomer