The code below helps to get a particular radio button checked on selecting a particular value from the dropdownlist.

<form>
    <div>
        <select id="selectvalue" onchange="Checkbox();">
            <option value="1">Select 1</option>
            <option value="2">Select 2</option>
            <option value="3">Select 3</option>
        </select>
        <input name="test" id="check1" value="1" type="radio">
        <input name="test" id="check2" value="2" type="radio">
        <input name="test" id="check3" value="3" type="radio">
    </div>
 </form>

<script type="text/javascript">

 function Checkbox() {
          // parseInt() is used to change the string into int.  
           var id = parseInt(document.getElementById('selectvalue').value);

            switch (id) {
                case 1:
                    document.getElementById("check1").checked = "checked";
                    break;
                case 2:
                    document.getElementById("check2").checked = "checked";
                    break;
                case 3:
                    document.getElementById("check3").checked = "checked";
                    break;
            }
}
</script>

Below is the generated output from the above code:

Select 1 Select 2 Select 3

<script type="text/javascript"> function Checkbox() { // parseInt() is used to change the string into int. var id = parseInt(document.getElementById('selectvalue').value); switch (id) { case 1: document.getElementById("check1").checked = "checked"; break; case 2: document.getElementById("check2").checked = "checked"; break; case 3: document.getElementById("check3").checked = "checked"; break; } } </script>