Queries :: Populating One Combo Box Based On Selection In Another

I can't quite get my code right for populating one combo box based on the selection in another. I didn't create this Access database but I've been asked to add in a second combo box that is based on the selection of the first combo box.

My first combo box is for Rohe (the location of the Maori tribe) and the second is Iwi (the tribe name). So my first combo box (Rohe) has Dvic_ROHE written in the control source and its row source code is:

SELECT tblLookUpValues.DatabaseFieldValue
FROM tblLookUpValues INNER JOIN tblFormQuestionLookUp ON tblLookUpValues.LookUpID = tblFormQuestionLookUp.LookUpID
WHERE (((tblFormQuestionLookUp.FormID)=2) AND ((tblFormQuestionLookUp.QuestionNumber)=75))
ORDER BY tblLookUpValues.DisplayOrder;

The combo box for Rohe (tribe location) is populated by a table named tblLookUpValues. The second combo box for Iwi (tribe) has Dvic_Iwi in the control source. I created a table named tbl_Rohe_Iwi_lookup that contains ROHE (a list of tribe locations) and IWI (the tribes). There are 12 tribal locations and another 10-14 tribes in each location.
The form that the Rohe and Iwi combo boxes are located on is called frmDeceased. This is the code that I attempted to write so that the second combo box (Iwi) is populated with only the tribes that are in the Rohe (location) that was selected from the Rohe combo box. But when I run the query it comes back with no results.

SELECT distinct tbl_Rohe_Iwi_lookup.Rohe, tbl_Rohe_Iwi_lookup.Iwi
FROM tbl_Rohe_Iwi_lookup
WHERE (((tbl_Rohe_Iwi_lookup.Rohe)=[forms]![frmDeceased]![Dvic_Rohe])) UNION select distinct null, null
FROM tbl_Rohe_Iwi_lookup
ORDER BY tbl_Rohe_Iwi_lookup.Iwi;