$(document).ready(function() {
    $("select[attribute=casetypecode]").change(function() {
        
        //value of the issue option
        var issueValue = 2;

        if (this.value == issueValue || this.value == "") {
            $("select[attribute=severitycode]").removeClass("read-only").each(function() { this.disabled = false });
        } else {
            $("select[attribute=severitycode]").addClass("read-only").each(function() { this.value = ""; this.disabled = true });
        }
    });
});