Automatically Selecting A Checkbox Based On The Value Of A Radio Button

I want to automatically select a specific checkbox when a user clicks
(selects) a specific item in a radiobutton group. Both controls are in
the same form.

Let's say for argument's sake that the form looks like this
(inessential items left out for the sake of clarity):

<form name=form1>
<input type=radio name=Radio1 value=Option1>
<input type=radio name=Radio1 value=Option2>
<input type=checkbox name=Checkbox1>
</form>

I want to write some Javascript to automatically select the "Checkbox1"
checkbox when a user selects "Option1" in the radio button group.