var categorieName="tip";
var subCategorieName="model";



function populateIt(){
  var prod=$("#"+categorieName).val(); 
  var sub=$("#"+subCategorieName);
  sub.removeOption(/./);
  sub.ajaxAddOption("ajax/php/generate.php?selectat="+prod);

}

$(document).ready(
    function(){
          $("#"+categorieName).bind('change',populateIt);
          $("#"+categorieName).trigger('change');  
        }
);