You are describing what we call a "conditional value list". Here are three links on the subject. The forum tutorial shows two options for setting up a conditional value list and either can work for what you describe. Option 1 is easier to set up, but less flexible.
Forum Tutorial: Custom Value List?
Knowledgebase article: http://help.filemaker.com/app/answers/detail/a_id/5833/kw/conditional%20value%20list