if( isset($_GET["state"]) {
     $where = $_GET['state'] == -1 ? "" : "state = '" . $_GET['state'] . "'";
}
if( isset($_GET["county"]) {
     $where .= $_GET['county'] == -1 ? "" : " AND county = '" . $_GET['county'] . "'";
}
// Variable editing should go before you use them.

$query = "Select DISTINCT Genus FROM Tennessee WHERE $where ORDER BY Genus";
mysql_query($query);