Multiple Master Detail Dropdown Lists Sorting

I have multiple drop down lists that I would like to have send parameters to a stored procedure that does a select on the data.  (Kind of like a search feature)  But if a ddl isnt populated, I want that to act like a * and get all.  For instance, I have a persontype, state, and country drop down lists.  If I drop the state one down to WA, I want the gridview to populate with all the people in WA regardless or their type.  If I populate type and state, have it give me both regardless of country.

What does the stored procedure or select syntax look like for this?  (What would the default values of my dropdown boxes?)

my current sql select statement (The bottom 3 are bit fields which are throwing me off a bit too).  do i need to have some IsNull statments around each variable in the select?SELECT

PersonFirstName
, PersonLastName
, PersonStreetAddress
, PersonCity
, PersonZip
, PersonEmail
, PersonEmailDate
, PersonPhone1
, PersonNotes
, InterestChapterLeaders
, InterestMembers
, InterestUniLeader
, PersonGradYear
, PersonTypeID
, CountryID
, StateID
, SchoolID
, PersonID

FROM tblPerson

WHERE

tblPerson.PersonTypeID = @PersonTypeID
and tblPerson.StateID = @StateID
and tblPerson.CountryID = @CountryID
and tblPerson.SchoolID = @SchoolID
and tblPerson.InterestChapterLeaders = @InterestChapterLeaders
and tblPerson.InterestMembers = @InterestMembers
and tblPerson.InterestUniLeader = @InterestUniLeader