Binding nested child gridview to master gridview with ObjectDataSources

I have two gridviews, one nested inside the other.  Both use objectdatasources for binding.  I am not sure how to take the value in the master gridview and place it in the paramenter for teh objectdatasource for the child gridview.

I've attached what my child objectdatasource looks like.  Right now the paramenter is set to the "SelectedValue", which gives me the same child gridview in each row.  I do I make it user the primary key of the master grid view?Start Free Trial

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<asp:ObjectDataSource ID="ObjectDataSource2" runat="server" 
        SelectMethod="GetCustomers" TypeName="DataObject2">
        <SelectParameters>
            <asp:ControlParameter ControlID="GridView1" Name="sItemType" 
                PropertyName="SelectedValue" Type="String" />
        </SelectParameters>
    </asp:ObjectDataSource>