Hi --
Add an expression box. For the value, use the secondary data source that provides the values to your Gender drop down list. Add a filter, so that Gender ID in your secondary drop down list is equal to the field in your main data source that is bound to the drop down fed by your secondary data source.
Samples speak louder than words, so see attached (right click link in header, select Save Target As to save locally. Right click file and select Design to open in design mode).