How to bind dropdownlist with checkbox to another dropdownlist selection

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C#

Ajax

jQuery

MVC5

I am trying to bind a checkboxlist on dropdownlist selection, i have been able to display a list on a multiselect dropdownlist on Lab selected index change, but adding a checkbox to the multiselect dropdown list is what i have not been able to do. Any assistance will be appreciated.

What I have tried:

My Ajax Function:

<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.8.3/jquery.min.js"></script>
<script type="text/jscript">
    $(document).ready(function () {
        $('#LabId').change(function () {
            $(".spina").show();
            $("#testType").prop("disabled", true);
            $.getJSON('/Lab/GetTestTypeList/' + $('#LabId').val(), function (data) {
                var items = '<option value = "0">---Select a Test Type---</option>';
                $.each(data, function (i, testType) {
                    items += "<option value='" + testType.Value + "'>" + testType.Text + "</option>";
                    $("#testType").prop("disabled", false);
                    $(".spina").hide();
                });
                $('#testType').html(items);
                $("#testType").prop("disabled", false);
                $(".spina").hide();
            });
        });
    });
</script>

My HTML Controls:

<div class="col-sm-7">
   <div class="form-group">
@Html.LabelFor(model => model.LabId, htmlAttributes: new { @class = "control-label col-md-2" })
<div class="col-md-10">
   @Html.DropDownListFor(model => model.LabId, (IEnumerable<SelectListItem>)ViewBag.LabList, "---Select Lab---", new { @class = "form-control" })
  @Html.ValidationMessageFor(model => model.LabId, "", new { @class = "text-danger" })
 </div>
 </div>
  </div>


 <div class="col-sm-7">
   <div class="form-group">
   @Html.LabelFor(model => model.TestTypeId, htmlAttributes: new { @class = "control-label col-md-2" })
   <div class="col-md-10">
      <select id="testType" name="testType" multiple="multiple" class="form-control" required="required">
   <option value="0">---Select a Test Type---</option>
     </select>
       @Html.ValidationMessageFor(model => model.TestTypeId, "", new { @class = "text-danger" })
     </div>
       </div>
         </div>

My C# Code:

private void LoadLabDropDownList()
        {
            try
            {
                var getList = repository.GetSpecialtyListByRoleId(8);
                var items = getList.Select(t => new SelectListItem
                {
                    Text = t.FullName,
                    Value = t.UserId.ToString()
                }).ToList();

                ViewBag.AccountantList = items;
            }
            catch (Exception ex)
            {
                ViewBag.DisplayMessage = "Info";
                ModelState.AddModelError("", ex.Message);
            }
}

The code to load the second dropdown list:

public JsonResult GetTestTypeList(string id)
        {
            if (!string.IsNullOrEmpty(id))
            {
                var labId = Convert.ToInt32(id);
                var testTypes = from s in db.TestTypes
                           where s.LabId == labId
                                select s;
                return Json(new SelectList(testTypes.ToArray(), "TestTypeId", "TestName"), JsonRequestBehavior.AllowGet);
            }
            else
            {
                var testTypeId = Convert.ToInt32(id);
                var testTypes = from s in db.TestTypes
                                where s.TestTypeId == 0
                                select s;
                return Json(new SelectList(testTypes.ToArray(), "TestTypeId", "TestName"), JsonRequestBehavior.AllowGet);
            }
        }

        [AcceptVerbs(HttpVerbs.Get)]
        public JsonResult LoadTestTypeByLabId(string labName)
        {
            var labList = GetTestType(Convert.ToInt32(labName));
            var labData = labList.Select(m => new SelectListItem()
            {
                Text = m.TestName,
                Value = m.TestTypeId.ToString(),
            });
            return Json(labData, JsonRequestBehavior.AllowGet);
        }

        public IList<TestType> GetTestType(int testTypeId)
        {
            return db.TestTypes.Where(m => m.TestTypeId == testTypeId).ToList();
        }