Are you saying you want to drive drop down appearance based on selection? That’s easy, you would use conditional visibility.
If the hope is to change the available options in the list, you should incorporate either the enumselector widget or the DynamicEnumPicker from the AppStore
If you are refering to the mechanism where choosing a value in DropDown1 should limit the contents of DropDown2, that is called selectable Objects, you can best check that subject in the documentation (Mendix has clear documentation). Example would be a specific country selected limits the cities you can choose from.
If you are looking for visual effect, check Marc's comment.