HTML
<select name="select1" id="select1" onChange="return changeForm();">
<option value="1">1</option>
<option value="2">2</option>
<option value="3">3</option>
<option value="4">4</option>
</select>
<select name="select2" id="select2">
</select>
JavaScript
function changeForm(){
var a = document.getElementById("select1");
var b = document.getElementById("select2");
var c = a.options[a.selectedIndex].value;
if(c == 1){
b.innerHTML = '<option value="1.1">1</option><option value="1.2">2</option>';
}
if(c == 2){
b.innerHTML = '<option value="2.1">1</option><option value="2.2">2</option>';
}
if(c == 3){
b.innerHTML = '<option value="3.1">1</option><option value="3.2">2</option>';
}
}