How to merge cells in WPF datagrid?

<DataGrid Name="DG" ItemsSource="{Binding Persons}"  AutoGenerateColumns="False" Width="Auto" SelectionMode="Single" SelectedItem="{Binding SelectedEmployee}" SelectedIndex="{Binding SelectedIndex}"  CanUserSortColumns="True" >

                          <DataGrid.Columns>
                              <DataGridTemplateColumn  Width="50"  IsReadOnly="True">
                                  <DataGridTemplateColumn.CellTemplate>
                                      <DataTemplate>

                                      </DataTemplate>
                                  </DataGridTemplateColumn.CellTemplate>
                              </DataGridTemplateColumn>
                              <DataGridTextColumn  Foreground="Black" Width="140"    Header="FirstName"   Binding="{Binding FirstName, Mode=TwoWay}" IsReadOnly="False"/>
                              <DataGridTextColumn Foreground="Black" Width="140" Header="LastName"  Binding="{Binding LastName, Mode=TwoWay}" IsReadOnly="False"/>
                              <DataGridTextColumn Foreground="Black" Width="140" Header="Age"  Binding="{Binding Age, Mode=TwoWay}" IsReadOnly="False"/>
                              <DataGridTextColumn Foreground="Black" Width="140" Header="Contact"  Binding="{Binding Contact, Mode=TwoWay}" IsReadOnly="False"/>

                          </DataGrid.Columns>



                      </DataGrid>