The first set of radio button controls are placed in a group with the same name rdoGroup1. When a group name is set to a set of radio buttons, the selection follows that. The first radio button in that group is by default selected, by setting its IsChecked property to True. If you select any other radio button within that group, the previous selection resets to unchecked status.
The same is true for the second group too, but selection of one group does not affect the other group. So, when you check one radio button from the first group, it will not uncheck the radio buttons from the other group.
This is not the same for CheckBox controls. Checkbox controls allow you to have many checked items. When you select a checkbox, it can just toggle from one state to another.
Both the radio button and checkbox control expose the IsChecked property to return a Boolean value to tell whether the control is checked or unchecked.