jQuery – checkbox enable/disable

Change your markup slightly:

$(function() {
  enable_cb();
  $("#group1").click(enable_cb);
});

function enable_cb() {
  if (this.checked) {
    $("input.group1").removeAttr("disabled");
  } else {
    $("input.group1").attr("disabled", true);
  }
}

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<form name="frmChkForm" id="frmChkForm">
  <input type="checkbox" name="chkcc9" id="group1">Check Me <br>
  <input type="checkbox" name="chk9[120]" class="group1"><br>
  <input type="checkbox" name="chk9[140]" class="group1"><br>
  <input type="checkbox" name="chk9[150]" class="group1"><br>
</form>

Expand snippet

You can do this using attribute selectors without introducing the ID and classes but it’s slower and (imho) harder to read.