code: def get_families_by_genus_type(self, family_genus_type=None):
          
          if family_genus_type is None:
              raise NullArgument()
          url_path = '/handcar/services/relationship/families'
          families_of_type = []
          all_families = self._get_request(url_path)
          for family in all_families:
              # DO WE NEED TO CHECK ALL THREE ATRIBUTES OF THE Id HERE?
              if family['genusTypeId'] == str(family_genus_type):
                  families_of_type.append(family)
          return objects.FamilyList(families_of_type)
docs: Gets a ``FamilyList`` corresponding to the given family genus ``Type`` which
              does not include families of genus types derived from the specified ``Type``.
  
          In plenary mode, the returned list contains all known families
          or an error results. Otherwise, the returned list may contain
          only those families that are accessible through this session.
  
          arg:    family_genus_type (osid.type.Type): a family genus type
          return: (osid.relationship.FamilyList) - the returned ``Family
                  list``
          raise:  NullArgument - ``family_genus_type`` is ``null``
          raise:  OperationFailed - unable to complete request
          raise:  PermissionDenied - authorization failure
          *compliance: mandatory -- This method must be implemented.*