Cascading drop down lists in Service Agreement form fields

I've tailored a Service Agreement form with various fields from the Service Agreement Entity adding some custom fields

I have the field of Registration Group with selectable options of A, B & C

I have a custom field of Service Type with drop down options of: AA1; AA2; BB1; BB2; CC1; CC2

I want to have a cascading drop down in the Service Type field after the Registration Group is selected.

How would I do this and where would I map the relationships between A with AA1 & AA2     B with BB1 & BB2      C with CC1 and CC2

Can I do this with a Business Rule?

cheers,

Dave