Cascading dropdown list using Ajax call

Well you can create an Application page with WebMethods defined and than use simple jQuery Ajax to call the methods..

Please have a look at
Call WebMethod from JS in Visual web Part

$.ajax({
     type: "POST",
     url: siteUrl + "/_layouts/SolutionName/WebMethods.aspx/Recount",
     data: "{'data':'" + data + "'}",
     contentType: "application/json; charset=utf-8",
     dataType: "json",
     async: true,
     success: function (data) {
         response(data["d"]);
     },
     error: function (result) {
         //alert("Error");
     }
 });

OR

You can simply use SPServices.Cascading Dropdown