I assume you need a "conditional value list". One of the people on the forum (PhilModJunk) has made a view examples how to deal with this issue.
If you type "conditional value list" in the forum you will find the examples.
This is a free to download FileMaker database file with working examples of 10 different implementations (Older version demo'd 8) of condtional value lists. You get working examples of each along with very detailed explanations of how each was set up and how they work.