function allCounty() {
 var strURL="displaydata6.php?state="+state;
req=makeRequest(strURL);
 document.getElementById('allcounty').innerHTML=req.responseText;
}

  <tr>
      <td>County</td>
      <td><div id="countydiv">
      <select name="county" onClick="genusSelect(this.value)">
      <option id="allcounty">Select All</option>
      </select>
      </div></td>
  </tr>