First, I don't understand why are you using DataGrid? You can create a much more simple solution using ItemsControl.
Second, You are trying to implement a Master-Detail pattern. The Combo-Box selected value is the Master and the TextBlock is the Details.
I've wrote a simple example that do exactly what you want to do in my article:
How to: Use the Master-Detail Pattern with WPF Hierarchical DataTemplate
Please review it. If you have more questions I'll be happy to help.