<DataGridTemplateColumn Header="Erledigt?" IsReadOnly="False">
<DataGridTemplateColumn.CellTemplate>
<DataTemplate>
<CheckBox Command="{Binding Path=DataContext.SetErledigtCommand,
RelativeSource={RelativeSource FindAncestor,
AncestorType={x:Type DataGrid}}}"
CommandParameter="{Binding ElementName=DgSource,
Path=SelectedItem}"
IsChecked="{Binding DataContext.Erledigt,
UpdateSourceTrigger=PropertyChanged}" />
</DataTemplate>
</DataGridTemplateColumn.CellTemplate>
</DataGridTemplateColumn>